Tokyo ( CNN ) -- Japan 's nuclear watchdog on Wednesday said a toxic water leak at the tsunami-damaged Fukushima Daiichi power plant has been classified as a level 3 `` serious incident '' on an international scale . The Nuclear Regulation Authority ( NRA ) said it had made the decision after consulting with the Vienna-based International Atomic Energy Agency , said Juntaro Yamada , a spokesman for the regulator . As news emerged last week of the leak of hundreds of tons of radioactive water from a storage tank , the NRA said it was planning to issue the alert , its gravest warning since the massive 2011 earthquake and tsunami that sent three reactors at the plant into meltdown . The leak had previously been assigned a level 1 `` anomaly rating '' on the International Nuclear and Radiological Event Scale , which ranges from zero , for no safety threat , to seven , for a major accident like the meltdowns . The decision to issue the level 3 alert came two days after a Japanese government minister had compared the plant operator 's efforts to deal with worrying toxic water leaks at the site to a game of `` whack-a-mole . '' Toshimitsu Motegi , the industry minister , said Monday after visiting the plant that `` from now on , the government is going to step forward . '' His ministry has been tasked by Prime Minister Shinzo Abe to come up with measures to tackle the mounting problems at Fukushima Daiichi . TEPCO looks for outside help to stabilize Fukushima . Huge volumes of toxic water . The plant operator , Tokyo Electric Power Company ( Tepco ) , has been struggling to deal with the high volume of contaminated water at the plant . Last month , Tepco admitted that radioactive groundwater was leaking into the Pacific Ocean from the site , bypassing an underground barrier built to seal in the water . About 400 tons of groundwater flow into the site each day , and Tepco also pumps large amounts water through the buildings to keep the crippled reactors cool . Fukishima tuna study finds miniscule health risks . The operator has stored hundreds of thousands of tons of the contaminated water in huge tanks at the site . There are now about 1,000 of the containers , 93 % of which are already full of radioactive water . Around 350 of the tanks were built as temporary storage units in the aftermath of the meltdowns . But more than two years later , they are still being used . It was one of those makeshift tanks where the leak was detected , setting off the latest crisis . Tepco says it has transferred the remaining tainted water from the faulty tank to another container . But it has n't said what caused the leak in the first place . Japan ponders freezing ground . CNN 's Junko Ogura reported from Tokyo , and Jethro Mullen wrote from Hong Kong .
The warning is Japan 's first warning since meltdowns at the nuclear plant in 2011 . The Japanese government made the announcement after consulting with the IAEA . A storage tank at the plant leaked hundreds of gallons of toxic water . A government minister this week criticized the plant operator 's handling of leaks .