SYDNEY The hunt for missing Malaysian Airlines Flight MH370 is on track to cost hundreds of millions of dollars, becoming the most expensive search in aviation history with 26 countries contributing planes, ships, submarines and satellites to the international effort. 
 
 A month into the search for the jet, estimates compiled by Reuters show that at least $44 million has already been spent on the deployment of military ships and aircraft in the Indian Ocean and South China Sea by Australia, China, the United States and Vietnam. The figure is based on defense force statistics on available hourly costs of various assets, estimates by defense analysts and costs reported by the Pentagon. 
 
 The figure for the first month of the search is already about equal to the official 32 million euros ($44 million) spent in searches lasting several months spread over a two-year time frame for Air France's Flight AF447, which crashed into the Mid-Atlantic in 2009. 
 
 Just as salvage experts said the actual costs for the Air France operation could have been three or four times higher than the official figure, the bill for the current search is expected to run into hundreds of millions of dollars. 
 
 The $44 million estimate for MH370 does not cover all the defense assets being used by countries including Britain, France, New Zealand and South Korea, nor numerous other costs such as civilian aircraft, accommodation for hundreds of personnel and expenses for intelligence analysts worldwide. 
 
 Britain dispatched a nuclear-powered submarine, HMS Tireless, to assist in the search, without disclosing the vessel's previous location. 
 
 Angus Houston, the retired Air Chief Marshal in charge of the Australian-led international search, said on Friday he would give an overall estimate of the cost at a later date. 
 
 "It's a lot of money," he said. 
 
 A FINANCIAL RECKONING 
 
 Australian Prime Minister Tony Abbott, whose country is leading the search, and his Malaysian counterpart, Najib Razak, have repeatedly said the cost of the search is not an issue. 
 
 Still, Abbott has hinted that Australia, which has so far borne the brunt of the expense as the search homes in on the southern Indian Ocean off its western coast, may at some point be sending out invoices. 
 
 "It's only reasonable that we should bear this cost - it's an act of international citizenship," Abbott said last week. 
 
 "At some point, there might need to be a reckoning, there might need to be some kind of tallying, but nevertheless we are happy to be as helpful as we can to all the countries that have a stake in this." 
 
 A Malaysian government source, who declined to be identified as he was not authorized to speak to the media, said the entire search and recovery for MH370 could be at least double the money spent to recover the black box from Air France's AF447. 
 
 Australia has so far contributed around half of the cost, with ships and aircraft on duty for some three weeks. Its HMAS Success alone costs around A$550,000 ($511,000) a day to operate, according to Australian Defence Force, or ADF, figures. 
 
 "The search effort for MH370 is costing the ADF at least A$800,000 per day and possibly considerably more than that," said Kym Bergmann, editor of Asia-Pacific Defence Reporter and a former government defence adviser. 
 
 "While that might not sound like much compared with the ADF's annual sustainment budget of A$5 billion, if this continues for much longer other activities such as 'Operation Sovereign Borders' will be under pressure to find savings," he added, referring to the federal government's policy of turning back asylum seeker boats that approach from Indonesia. 
 
 CHINA, U.S. SPENDING 
 
 The other big spenders so far are China and the United States. 
 
 China, which was home to the majority of the 227 passengers on board the missing plane, has sent a total of 18 ships, eight helicopters and three fixed-wing aircraft to various search areas during the month-long hunt. 
 
 Beijing has declined to comment on how much it is spending on looking for the aircraft, saying only it is dedicated to keep searching "as long as there is a shred of hope". 
 
 China has been sending two Ilyushin Il-76 aircraft, on alternate days, on sorties from RAAF Pearce Base near Perth for almost three weeks. The Global Times, an influential tabloid published by the ruling Communist Party's official People's Daily, estimated that an Ilyushin Il-76 costs $10,000 an hour to keep in the air on fuel alone, not including money spent on maintenance or accommodation for the crews. 
 
 Chinese warships would cost at least $100,000 a day to operate, and most likely a lot more, the newspaper added. 
 
 "There's a lot of pressure in China to find the plane," said a Beijing-based Western diplomat. "China will spare no effort." 
 
 The Pentagon said last week that it had already spent more than $3.3 million on the search and has put in place plans to nearly double its original $4 million budget. 
 
 As well as flying its P-8 surveillance planes out of Perth, the U.S. Navy is playing an instrumental role via its high-tech underwater black-box detector equipment. 
 
 It has sent both its Towed Pinger Locator, which this week picked up signals which may be from the missing plane's cockpit data recorders, and a Bluefin-21 autonomous underwater vehicle. 
 
 An earlier search in the South China Sea by Vietnam was estimated by local media to cost $8 million, a figure that has not been verified by officials. 
 
 ($1 = 1.0759 Australian Dollars) ($1 = 0.7303 Euros) 
 
 (Additional reporting by Niluski Koswanage in KUALA LUMPUR, Ben Blanchard in BEIJING, Swati Pandey in PERTH and Mai Nguyen in HANOI; Editing by Lincoln Feast and Kenneth Maxwell) ||||| PERTH, Australia (AP) — Search crews in the Indian Ocean failed to pick up more of the faint underwater sounds that may have been from the missing Malaysian jetliner's black boxes whose batteries are at the end of their life. 
 
 In this April 7, 2014 photo provided by the Australian Defense Force a fast response craft manned by members from the Australian Defense's ship Ocean Shield is deployed to scan the water for debris of... (Associated Press) 
 
 In this April 7, 2014 photo provided by the Australian Defense Force a fast response craft manned by members ofthe Australian Defense's ship Ocean Shield is deployed to scan the water for debris of the... (Associated Press) 
 
 Flight crew from a Royal Australian Air Force P-3 Orion walk towards the aircraft at RAAF Base Pearce as it arrives back from the on-going search operations for missing Malaysia Airlines Flight 370 in... (Associated Press) 
 
 Feng Zhishang, right, cries as family members mark the birthday of his son Feng Dong, a passenger on board the Malaysia Airlines Flight 370 at a hotel where relatives gather to wait for news of the missing... (Associated Press) 
 
 In this April 7, 2014 photo provided by the Australian Defense Force, a fast response craft manned by members from the Australian Defense's ship Ocean Shield tows Able Seaman Clearance Diver Michael Arnold... (Associated Press) 
 
 In this April 7, 2014 photo provided by the Australian Defense Force, a fast response craft manned by members of the Australian Defense's ship Ocean Shield is deployed to scan the water for debris of... (Associated Press) 
 
 In this April 7, 2014 photo provided by the Australian Defense Force Able Seaman Clearance Divers Matthew Johnston, right, and Michael Arnold, from the Australian Defense Vessel Ocean Shield, scan the... (Associated Press) 
 
 In this April 7, 2014 photo provided by the Australian Defense Force Able Seaman Clearance Diver Michael Arnold is towed by a fast response craft from the Australian Defense's ship Ocean Shield as he... (Associated Press) 
 
 Feng Zhishang, second from right, cries as family members celebrate the birthday for his son Feng Dong, a passenger on board the Malaysia Airlines Flight 370 at a hotel where relatives gather to wait... (Associated Press) 
 
 The signals first heard late Saturday and early Sunday had sparked hopes of a breakthrough in the search for Flight 370, but Angus Houston, the retired Australian air chief marshal leading the search far off western Australia, said listening equipment on the Ocean Shield ship has picked up no trace of the sounds since then. 
 
 Finding the sound again is crucial to narrowing the search area so a submarine can be deployed to chart a potential debris field on the seafloor. If the autonomous sub was used now with the sparse data collected so far, covering all the potential places from which the pings might have come would take many days. 
 
 "It's literally crawling at the bottom of the ocean so it's going to take a long, long time," Houston said. 
 
 The locator beacons on the black boxes have a battery life of only about a month — and Tuesday marked exactly one month since the plane vanished. Once the beacons blink off, locating the black boxes in such deep water would be an immensely difficult, if not impossible, task. 
 
 "There have been no further contacts with any transmission and we need to continue (searching) for several days right up to the point at which there's absolutely no doubt that the batteries will have expired," Houston said. 
 
 If, by that point, the U.S. Navy towed pinger locator has failed to pick up more signals, the sub will be deployed. If it maps out a debris field on the ocean floor, the sonar system on board will be replaced with a camera unit to photograph any wreckage. 
 
 Earlier, Australia's acting prime minister, Warren Truss, had said the Bluefin 21 autonomous sub would be launched on Tuesday, but a spokesman for Truss said later the conflicting information was a misunderstanding, and Truss acknowledged the sub was not being used immediately. 
 
 Houston earlier said the two sounds heard Saturday and Sunday are consistent with the pings from an aircraft's black boxes. 
 
 Defense Minister David Johnston called the sounds the most positive lead and said it was being pursued vigorously. Still, officials warned it could take days to determine whether the sounds were connected to the plane that vanished March 8 on a flight from Kuala Lumpur, Malaysia, to Beijing with 239 on board. 
 
 "This is an herculean task — it's over a very, very wide area, the water is extremely deep," Johnston said. "We have at least several days of intense action ahead of us." 
 
 Houston also warned of past false leads — such as ships detecting their own signals. Because of that, other ships are being kept away, so as not to add unwanted noise. 
 
 "We're very hopeful we will find further evidence that will confirm the aircraft is in that location," Houston said. "There's still a little bit of doubt there, but I'm a lot more optimistic than I was one week ago." 
 
 Such optimism was overshadowed by anguish at a hotel in Beijing where around 300 relatives of the flight's passengers — most of whom were Chinese — wait for information about the plane's fate. 
 
 One family lit candles on a heart-shaped cake to mark what would have been the 21st birthday of passenger Feng Dong, who had been working in construction in Singapore for the past year and was flying home to China via Kuala Lumpur. Feng's mother wept as she blew out the candles. 
 
 A family member of another passenger said staying together allowed the relatives to support one another through the ordeal. "If we go back to our homes now it will be extremely painful," said Steve Wang. "We have to face a bigger pain of facing uncertainty, the unknown future. This is the most difficult to cope with." 
 
 Investigators have not found any explanation yet for why the plane lost communications and veered far off its Beijing-bound course, so the black boxes containing the flight data and cockpit voice recorders are key to learning what went wrong. 
 
 "Everyone's anxious about the life of the batteries on the black box flight recorders," said Truss, who is acting prime minister while Tony Abbott is overseas. "Sometimes they go on for many, many weeks longer than they're mandated to operate for — we hope that'll be the case in this instance. But clearly there is an aura of urgency about the investigation." 
 
 The first sound picked up by the equipment on board the Ocean Shield lasted two hours and 20 minutes before it was lost, Houston said. The ship then turned around and picked up a signal again — this time recording two distinct "pinger returns" that lasted 13 minutes. That would be consistent with transmissions from both the flight data recorder and the cockpit voice recorder. 
 
 The black boxes normally emit a frequency of 37.5 kilohertz, and the signals picked up by the Ocean Shield were both 33.3 kilohertz, U.S. Navy Capt. Mark Matthews said. 
 
 Houston said the frequency heard was considered "quite credible" by the manufacturer, and noted that the frequency from the Air France jet that crashed several years ago was 34 kilohertz. The age of the batteries and the water pressure in the deep ocean can affect the transmission level, he said. 
 
 The Ocean Shield is dragging a pinger locator at a depth of 3 kilometers (1.9 miles). It is designed to detect signals at a range of 1.8 kilometers (1.12 miles), meaning it would need to be almost on top of the recorders to detect them if they were on the ocean floor, which is about 4.5 kilometers (2.8 miles) deep. 
 
 The surface search for any plane debris also continued Tuesday. Up to 14 planes and as many ships were focusing on a single search area covering 77, 580 square kilometers (29,954 square miles) of ocean, said the Joint Agency Coordination Center, which is overseeing the operation. 
 
 ___ 
 
 Associated Press writers Kristen Gelineau in Sydney, Rod McGuirk in Canberra, Australia, and video journalists Isolda Morillo and Peng Peng in Beijing contributed to this report.
– As searchers listen for pings in the Indian Ocean that might help them find the missing Flight 370, Reuters assesses the cost of the hunt after a month and finds that it's on track to become the most expensive search in aviation history. Reuters figures that the militaries of Australia, China, the US, and Vietnam have spent $44 million already, about the amount spent in the two-year search for the Air France jet that went missing in 2009. The $44 million total doesn't take into account civilian aircraft or the cost of accommodating hundreds of intelligence analysts and other personnel worldwide. Add those expenses in, and the total is in the "hundreds of millions of dollars." Australia is leading the search and has borne about half the costs so far, though Prime Minister Tony Abbott said the nation may ask others to defray its expenses later. "At some point, there might need to be a reckoning, there might need to be some kind of tallying, but nevertheless we are happy to be as helpful as we can to all the countries that have a stake in this," he said. The Pentagon estimates that the US has spent $3.3 million so far. With no sign of the pings in two days, the search could drag on and on, reports AP. Australia plans to release a submersible to scan the ocean floor, but it hopes to get a more precise location before doing so.