VIENNA (AP) _ The situation at Japan's tsunami-stricken nuclear plant is ``very serious,'' but at the moment does not appear to be deteriorating, a senior official of the U.N. atomic agency said Thursday.

As emergency workers frantically worked to regain control of the dangerously overheated nuclear complex, Graham Andrew told reporters ``there had been no significant worsening'' over the past 24 hours at the crippled plant.

Andrew, a senior aide to International Atomic Energy Agency chief Yukiya Amano, emphasized that the situation could change quickly, either improving or escalating into a wider catastrophe.

``It hasn't gotten worse, which is positive, But it is still possible that it could get worse,'' he said. ``We could say it's reasonably stable at the moment compared to yesterday.''

Andrew appeared to be the first senior IAEA official to say that the situation has not significantly deteriorated since the agency began briefings Monday. On Wednesday, Amano had described the situation as ``very serious,'' adding ``it is difficult to say'' if events were spinning out of control.

Andrew spoke shortly after Amano flew to Tokyo to assess efforts to fight the nuclear havoc unleashed by the massive earthquake and tsunami that hit Japan's northeastern coast Friday.

It was unclear what Amano hoped to accomplish during his one-day trip; he has said he plans to stay in Tokyo and meet with government officials but he had no agenda or scheduled meetings before takeoff from Vienna international airport.

``We don't have a fixed schedule and don't have all the information so we will be thinking on our feet,'' Amano told reporters assembled in the departure hall.

Still, he suggested his trip was symbolically important as his home country wrestles with its worst nuclear crisis since the bombing of Hiroshima and Nagasaki 66 years ago.

``Japan is not alone, the international community is standing by Japan,'' Amano declared. ``We have lots of offers of assistance to Japan and I would like to convey this message to them.''

In Japan, military helicopters dumped loads of sea water onto the Fukushima Dai-ichi plant Thursday as they tried to cool overheated uranium fuel that may be on the verge of spewing out more radiation.

As the crisis unfolds in Japan, the IAEA has been under increased scrutiny as to whether it has done enough to increase the safety culture in the nuclear reactor industry.

A U.S. Embassy cable from three years ago on the WikiLeaks website published Thursday by Britain's ``The Guardian'' daily cited criticism of Tomihiro Tamniguchi a senior IAEA official in charge of nuclear safety and security, who stepped down two years ago.

It described him as a ``weak manager ... particularly with respect to confronting Japan's own safety practices.''

The IAEA declined comment, and officials at Thursday's briefing sidestepped specific questions on how the agency's safety recommendations applied to the Fukushima Dai-ichi plant.

But while conceding the enormity of the Japan disaster, Andrew defended the ``enviable safety record'' of the nuclear sector in terms of deaths compared to ``other ways of creating energy.''

A graphic from another U.N. agency forecasting the possible trajectory of the radioactive cloud shows a moving plume reaching Southern California on Friday after racing across the Pacific and swiping the Aleutian Islands.

But IAEA officials and independent experts emphasized that the radiation level was already low outside of the immediate vicinity of the crippled reactor and would dissipate so strongly by the time it reached the U.S. coastlines that it would pose no health risk whatsoever to residents there.

Any detectable radiation on Friday ``could be coming from your own reactors in California,'' said physics Prof. Paddy Regan at the University of Surrey at Guilfold in Britain. A senior IAEA official who asked for anonymity because he was not authorized to speak to media, noted that even in Tokyo, ``radiation was at background levels.''

He said that _ while the moving schematic drawn up by the Comprehensive Test Ban Treaty Organization and shared with The Associated Press was useful for meteorological purposes _ it had no relevance for radiation measurements.