Sebastian Pflugbeil, president of the private German-based Society for Radiation Protection, said Japan's efforts to pull the Fukushima 1 plant back from the brink signalled "the beginning of the catastrophic phase".

"Maybe we have to pray," he said.

The head of the US Nuclear Regulatory Commission, Gregory Jaczko, said he believes the situation is more serious than the Japanese government is letting on.

Mr Jaczko warned water in reactor 4's cooling pool may have run dry and a second reactor could be leaking - something experts say could accelerate the release of radiation.

"We believe that around the reactor site there are high levels of radiation," he said.

"It would be very difficult for emergency workers to get near the reactors.

The doses they could experience would potentially be lethal doses in a very short period of time.

The IAEA said that the reactor 4 cooling pool has trebled in temperature while those for reactors 5 and 6 have more than doubled.

But Japan's nuclear safety agency and Tepco denied Mr Jaczko's statement that the water is gone from the cooling pool.

Utility spokesman Hajime Motojuku said reactor 4's condition is "stable".

Four people were injured in the blast at reactor 1 on March 11, and at least 11 others were hurt in a subsequent explosion at reactor 3 on March 14.

According to the IAEA, two people are still missing and at least 20 workers, police and firemen have suffered radiation contamination.

Meanwhile, thousands of people have been cleared from their homes, and thousands more have chosen to move away from the region surrounding the Fukushima plant which is 150 miles (240km) north of Tokyo.

Both the UK and US governments have warned citizens living within 50 miles (80 km) of the reactors to evacuate.

Japan said the US would fly a high-altitude drone over the stricken complex to gauge the situation with infrared cameras and other monitoring devices.

The rapidly-changing situation has also led Britain, Germany, France, Austria and Australia to urge its citizens to leave the country's capital.

The UK government is chartering flights from Tokyo to Hong Kong to supplement commercially available options for those wishing to leave.

The Foreign Office has advised against any non-essential travel to Tokyo or the quake-hit area.

Asian and European airlines have begun diverting Tokyo flights to Osaka or cancelling them altogether.

The Vienna-based IAEA director general is travelling to Japan to assess what he called a "very serious" situation and urged the Japanese government to provide better information to his organisation.

China has urged Japan to give "accurate" and swift information on the nuclear crisis.

The US-based Associated Press has also revealed that behind the escalating nuclear crisis sits a scandal-ridden energy industry in a comfy relationship with government regulators often willing to overlook safety lapses.

Japan's trade minister has also warned that Tokyo may suffer rolling power blackouts if energy demands rise too high.