Story highlights Two Americans unaccounted for, senior U.S. official says 
 
 Obama says "scourge of terrorism" must be fought 
 
 Algerian state news: At least 55 people, including hostages and militants, have died 
 
 The militants targeted an eastern Algerian gas facility where many foreigners workers 
 
 Algerian troops ended a hostage crisis at a remote gas facility Saturday with one last, bloody assault, Algerian and Western officials said, after three days of chaos and confusion left dozens dead and fanned fears of a new terror front in Africa. 
 
 At least 23 hostages and 32 "terrorists" were killed around the sprawling facility in eastern Algeria's desert, the Algerian interior ministry said Saturday. Some 685 Algerian workers and 107 foreigners have been freed, it said. 
 
 It is not clear just how many people are still unaccounted for. A senior U.S. official said two Americans were among that number. 
 
 The saga closed after a "final" assault, which itself contributed to the deaths of seven hostages and 11 militants, according to Algerian state media reports. 
 
 An Algerian Radio report did not specify the nationalities of those killed. CNN is unable to verify the state media figures on the deaths. 
 
 Afterward, Algeria's military continued to clear mines planted by militants, the official Algerian Press Service reported, citing the country's state-owned oil and gas company. 
 
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 "While the site is still dangerous and there may be explosives that will need to be dealt with, the terrorist incident is now over," said British Prime Minister David Cameron, citing his conversation with his Algerian counterpart. 
 
 The militant siege caught the world's attention as it ensnared citizens from several nations and dragged on for days. 
 
 Algerian authorities said they believe the attack was revenge for allowing France to use Algerian airspace for an offensive against Islamist militants in neighboring Mali. 
 
 Whatever the rationale, the scale and gore of the terror has stirred world leaders to press for action beyond Algeria, especially with Islamic extremists asserting themselves more and more in recent weeks. 
 
 "Let me be clear: There is no justification for taking innocent life in this way," Cameron said. "Our determination is stronger than ever to work with allies ... around the world to root out and defeat this terrorist scourge and those who encourage it." 
 
 British Foreign Secretary William Hague said Algerian authorities "certainly worked very hard to try to save the lives of people from other countries. I think it's actually too early to pass judgment on (the Algerian military operation)." 
 
 U.S. President Barack Obama said his administration would work with other countries to "combat the scourge of terrorism in the region, which has claimed too many innocent lives. This attack is another reminder of the threat posed by al Qaeda and other violent extremist groups in North Africa." 
 
 In a statement, Obama said the United States will work with Algeria to prevent future such incidents. 
 
 Crisis highlights pressure, potential on Algeria 
 
 Algerian special forces moved in because the terrorists wanted to flee for Mali -- apparently to pressure France and others who have recently intervened in that country -- Algerian state TV reported Saturday. 
 
 The Islamic extremists also planned to blow up the gas installation, a threat that initially prompted Algeria to halt its military operation. The location was rigged to explode, with mines planted throughout, according to the senior U.S. official. 
 
 Algeria's status as Africa's largest natural gas producer, and as a major supplier of the product to Europe, heightens its importance to other nations and businesses who want to invest there. Yet that interest is coupled with pressure to make sure foreign nationals, and their business ventures, are safe. 
 
 The targeted plant in In Amenas, which is just 50 kilometers (30 miles) west of the Libyan border, is run by Algeria's state oil company, in cooperation with foreign firms such as Norway's Statoil and Britain's BP. 
 
 Saturday's last push there follows another one two days earlier, which spurred criticism from some countries that Algeria had unnecessarily endangered hostages' lives. 
 
 British Defense Secretary Philip Hammond said Britain was still pressing the Algerian government for full details, while confirming the Saturday assault resulted in more deaths. He called the loss of life "appalling and unacceptable," laying blame solely on the terrorists. 
 
 Like Cameron, Norwegian foreign ministry spokesman Svein Michelsen confirmed that the Algerian military offensive was over, but did not offer further details. 
 
 Nations scramble to account for missing 
 
 The overall death toll could rise, Algerian state TV reports, as authorities are still combing the area. 
 
 Amid the uncertainty, individual nations are scrambling to find out what happened to their citizens. It is not clear how many hostages were seized by the Islamist militants in the first place. 
 
 Five Norwegians are missing while "eight are now safe," Norwegian Prime Minister Jens Stoltenberg said. 
 
 Five British nationals and one UK resident are missing or feared dead in Algeria, Hague told reporters. This is in addition to one Briton, whose death was previously announced. 
 
 Colombia's president said a citizen was presumed dead. 
 
 The Scottish government said eight of its residents are safe. 
 
 There are no known French hostages unaccounted for, a Defense Minister Jean-Yves Le Drian said Saturday. 
 
 Three French nationals who were at the site are safe, the foreign ministry has said. One man -- identified as Yann Desjeux -- died after telling the French newspaper Sud Ouest on Thursday that he and 34 other hostages of nine different nationalities were well-treated. 
 
 Of the BP employees, 14 are safe, and four BP employees are still missing in Algeria, BP Chief Executive Bob Dudley said. 
 
 At least one American, identified as Frederick Buttaccio, is among the dead, U.S. State Department spokeswoman Victoria Nuland said. The senior U.S. official told CNN that six freed Americans left Algeria and one remained. 
 
 One Romanian lost his life, a spokeswoman for the Romanian Ministry of Foreign Affairs told CNN on Saturday. Four other Romanians were freed. 
 
 And there are 14 Japanese unaccounted for, Chief Cabinet Secretary Yoshihide Suga said. 
 
 Malaysia's state-run news agency reported Thursday that two of its citizens were held captive. 
 
 Dramatic tales of escape from terror 
 
 When the crisis began Wednesday, militants gathered the Westerners into a group and tied them up, survivors said. The kidnappers wielded AK-47 rifles and put explosive-laden vests on some hostages, according to a U.S. State Department official. 
 
 Some survivors described their harrowing escapes by rigging up disguises, sneaking to safety with locals, and in at least one case, running for his life with plastic explosives strapped around his neck. 
 
 That man was Stephen McFaul, who -- according to his brother Brian -- was among a group of hostages who had been blindfolded, gagged and then packed into five Jeeps on Thursday, during Algerian forces' first offensive. 
 
 An explosion "wiped out" four of the vehicles, while McFaul's vehicle crashed. He was able to get out and, eventually, contact his family. 
 
 "I haven't seen my mother move as fast in all my life, and my mother smile as much, hugging each other," Brian McFaul of Belfast, Northern Ireland, said upon his family hearing his brother was safe. "... You couldn't describe the feeling." 
 
 Al Qaeda-linked group offered prisoner-hostage exchange 
 
 A spokesman for Moktar Belmoktar , a longtime jihadist who leads the Brigade of the Masked Ones -- a militant group associated with al Qaeda in the Islamic Maghreb -- reportedly offered to free U.S. hostages in exchange for two prisoners. 
 
 Behind the group claiming responsibility for the attack and kidnappings, he is known for seizing hostages and has long been targeted by French counterterrorism forces. 
 
 The prisoners are Sheikh Omar Abdel Rahman, who orchestrated the 1993 World Trade Center bombing, and Aafia Siddiqui , a Pakistani woman jailed in the United States on terrorism charges, the spokesman said in an interview with a private Mauritanian news agency. 
 
 Nuland rejected the offer, restating U.S. policy of not negotiating with terrorists. ||||| BAMAKO, Mali — The four-day hostage crisis in the Sahara reached a bloody conclusion on Saturday as the Algerian Army carried out a final assault on the gas field taken over by Islamist militants, killing most of the remaining kidnappers and raising the total of hostages killed to at least 23, Algerian officials said. 
 
 Although the government declared an end to the militants’ siege, the authorities believed that a handful of jihadists were most likely hiding somewhere in the sprawling complex and said that troops were hunting for them. 
 
 The details of the desert standoff and the final battle for the plant remained murky on Saturday night — as did information about which hostages died and how — with even the White House suggesting that it was unclear what had happened. In a brief statement released early Saturday night the president said his administration would “remain in close touch with the government of Algeria to gain a fuller understanding of what took place.” 
 
 The British defense minister, Philip Hammond, called the loss of life “appalling and unacceptable” after reports that up to seven hostages were killed in the final hours of the hostage crisis, and he said that the leaders of the attack would be tracked down. The Algerian government said that 32 militants had been killed since Wednesday, although it cautioned that its casualty counts were provisional. 
 
 Defense Secretary Leon E. Panetta, who appeared with Mr. Hammond at a news conference in London, said he did not yet have reliable information about the fate of the Americans at the facility, although a senior Algerian official said two had been found “safe and sound.” 
 
 What little information trickled out was as harrowing as what had come in the days before, when some hostages who had managed to escape told of workers being forced to wear explosives. They also said that there were several summary executions and that some workers had died in the military’s initial rescue attempt. 
 
 On Saturday, Algerian officials reported that some bodies found by troops who rushed into the industrial complex were charred beyond recognition, making it difficult to distinguish between the captors and the captured. Two were assumed to be workers because they were handcuffed. 
 
 Britain’s foreign secretary, William Hague, said that five Britons and one British resident had died in the final rescue attempt or were unaccounted for. He said that police forces were fanning out across Britain visiting each of the families involved. 
 
 Most of the hundreds of workers at the plant, who come from about 25 countries, appear to have escaped sometime during the four days. 
 
 The Algerian government has been relatively silent since the start of the crisis, releasing few details. The government faced withering international criticism for rushing ahead with its first assault on the militants on Thursday even as governments whose citizens were trapped inside the plant pleaded for more time, fearing that rescue attempts might lead to workers dying. The Algerians responded by saying they had a better understanding of how to handle militants after fighting Islamist insurgents for years. 
 
 On Saturday, it was unclear who killed the last hostages. Initial reports from Algerian state news media said that seven workers had been executed during the army’s raid, but the senior government official and another high-level official, both of whom spoke on the condition of anonymity, later said the number killed and the cause were unknown. The early reports also said 11 militants were killed, but later information suggested that some may have blown themselves up. 
 
 One of the Algerian officials defended the latest military assault, saying the government feared the militants were about to set off explosions at the In Amenas complex. 
 
 The Algerian state oil company, Sonatrach, said that the attackers had evidently mined the facility with the intention of blowing it up and that the company was working to ensure the safety of the plant. 
 
 The government official, meanwhile, said that the militants had set fire to the plant’s control tower on Friday night and that it was later extinguished by soldiers and workers. The militants also tried to blow up a pipeline, he said, leading officials to worry about the stocks of gas at the plant. “The authorities were afraid they were going to blow up the reserves,” said the official, who believed the militants had planned all along to destroy the complex. 
 
 Reporting was contributed by Steven Erlanger and Scott Sayare from Paris, Elisabeth Bumiller and John F. Burns from London, Manny Fernandez and Clifford Krauss from Houston, and Michael R. Gordon from Washington.
– The Algerian hostage standoff appears to be over after four days, but the end came with more bloodshed: The government says kidnappers executed seven hostages during this morning's "final assault." That brings the death toll to at least 32 "terrorists" and 23 hostages, including one American, report AP and CNN. No details are out yet on nationalities of the latest victims. "In principle, it's over," a government official tells the New York Times. Only a "clean-up process" remains. A Turkish official says two remaining American hostages were found "safe and sound." Algeria's state news agency reports that another 11 of the al-Qaeda-linked militants were killed in the raid, but it did not say whether any remained alive. As for that "clean-up process": The militants reportedly booby-trapped the entire gas refinery with explosives, and Algerian troops had begun the painstaking task of clearing them.