BRUSSELS - Japan's nuclear crisis remains very serious but there are some signs of progress, the head of the U.N. atomic agency said on Thursday.

Japan this week unveiled new plans to contain the crisis at the crippled Fukushima nuclear plant after admitting it faced greater challenges than first disclosed. But it kept to a goal of bringing the reactors under control by January.

"It (the Fukushima nuclear emergency) is a very serious accident and it still continues," said Yukiya Amano, director general of the International Atomic Energy Agency (IAEA).

"The situation continues to be very serious but we have some signs of recovery, like the recovery of electricity or instrumentation, and the government of Japan has just announced a plan to stabilise the reactors," Amano told reporters.

Earlier on Thursday, operator Tokyo Electric said workers had entered the last of three reactor buildings hit by nuclear fuel meltdowns at Fukushima, which has been leaking radiation for more than two months.

The utility's efforts to stabilise the plant suffered a further setback this week when it learned of leaks in the three reactors' pressure vessels.