THE operator of Japan's wrecked Fukushima nuclear plant was careless in monitoring tanks storing dangerously radioactive water, the country's nuclear regulator said, the latest development in a crisis no-one seems to know how to contain.

Tokyo Electric Power Company also failed to keep records of inspections of the tanks, Nuclear Regulation Authority (NRA) Commissioner Toyoshi Fuketa said after a visit to the nearby Fukushima Dai-ichi plant.

Mr Fuketa visited the plant yesterday after NRA chairman Shunichi Tanaka said he was concerned that more containers would fail. "They did not take action that foresaw the risks of possible leaks," Mr Fuketa said.

"I can't help but say that the inspections were careless."

Japan's nuclear crisis this week escalated to its worst level since a massive earthquake and tsunami crippled the plant in 2011.