A Japanese woman is screened for radiation after being evacuated from contaminated areas around the Fukushima nuclear plant, which was damaged by last week's major earthquake and tsunami.

Tokyo - Operators of a quake-crippled nuclear plant in Japan said they would try again on Thursday to use military helicopters to douse overheating reactors and avert a disaster, but United States officials warned that radiation levels may be too high to allow repairs.

While officials scrambled to contain the nuclear crisis with a variety of patchwork fixes, health experts said panic over radiation leaks from the Daiichi plant may divert attention from potentially worse threats to survivors of Friday's 9.0 magnitude quake and tsunami, such as the cold or access to fresh water.

The head of the world's nuclear watchdog, meanwhile, said while it was not accurate to say things were “out of control” in Japan, the situation was “very serious”, with core damage to three units at the plant.

A steady stream of gloomy warnings and reports on the Japan crisis from experts and officials around the world triggered something of a meltdown in US markets, with all three major US stock indexes slumping over 2 percent on fears of slower worldwide growth.

Traders were glued to their screens, hitting the sell button every time officials gave ever bleaker assessments of the situation on the ground in Japan.

The top US nuclear regulator told Congress that radiation levels around Japan's troubled nuclear power plant may give emergency workers lethal doses of radiation, preventing them from getting near the plant.

“We believe that around the reactor site there are high levels of radiation,” Gregory Jaczko, head of the Nuclear Regulatory Commission, told a House Energy and Commerce subcommittee hearing.

“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.”

Japan's government said radiation levels outside the plant's gates were stable but, in a sign of being overwhelmed, appealed to private companies to help deliver supplies to tens of thousands of people evacuated from around the complex.

Bulldozers attempted to clear a route to the reactor, about 240km from Tokyo, so firetrucks could gain access and try to cool the facility using hoses.

“People would not be in immediate danger if they went outside with these levels. I want people to understand this,” Chief Cabinet Secretary Yukio Edano told a news conference, referring to people living outside a 30km exclusion zone.

High radiation levels on Wednesday prevented a helicopter from dropping water into the No. 3 reactor to try to cool its fuel rods after an earlier explosion damaged the unit's roof and cooling system.

Officials from the Tokyo Electric Power Company said shortly after midnight that they would ask the military to make a second attempt later on Thursday.

The plant operator described No. 3 - the only reactor at that uses plutonium in its fuel mix - as the “priority”. Plutonium, once absorbed in the bloodstream, can linger for years in bone marrow or liver cells and lead to cancer.

The situation at No. 4 reactor, where the fire broke out, was “not so good”, the plant operator added, while water was being poured into reactors No. 5 and 6, indicating the entire six-reactor facility was now at risk of overheating.

“Getting water into the pools of the No.3 and No.4 reactors is a high priority,” Said Hidehiko Nishiyama, a senior official at Japan's Nuclear and Industrial Safety Administration, adding the pool for spent fuel rods at No. 3 was heating up while No.4 remained a concern.

“It could become a serious problem in a few days,” he said. - Reuters