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The head of a U.S. nuclear watchdog agency said Wednesday the threat posed by Japan reactors that have been damaged by the earthquake and tsunami is greater than depicted.

The New York Times reported U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko told U.S. lawmakers the damage at one damaged reactor was more severe than Japanese officials had disclosed and advised Americans to get further away from the plant.

Jaczko testified the commission believed all the water in the spent fuel pool at the No. 4 reactor of the Fukushima Daiichi Nuclear Power Station had boiled off, leaving fuel rods stored exposed and leaking radiation, the Times said.

"We believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," he said.

He said radiation levels could complicate cooling procedures that have helped keep the fuel from melting at the other reactors.

The newspaper said the U.S. Embassy in Tokyo has told Americans to evacuate a radius of "approximately 50 miles" from the Fukushima plant.

U.S. Energy Secretary Steven Chu told a House panel Wednesday "we think there is a partial meltdown."

"We hear conflicting reports about exactly what is happening in the several reactors now at risk," he said. "I would not want to speculate about what is happening."

Earlier in the day, Japanese officials said a second reactor unit at the plant may have sustained damage to its primary containment structure and appeared to be releasing radioactive steam.

The break, at the No. 3 reactor unit came a day after officials said the containment vessel in the No. 2 reactor had also cracked.

High radiation levels above the plant led the Japanese military to hold off on a plan to dump water on the facility with helicopters.

The Pentagon said Wednesday American military forces were not allowed within 50 miles of the plant.

White steam was seen rising above the stricken facility, which was damaged by Friday's 9-magnitude earthquake and tsunami and suffered a series of explosions. Military officials decided against exposing helicopter crews to the radiation.

Japanese officials have attempted to tamp down panic, saying the suspected rupture probably was not severe, the Times reported.

Tokyo Electric Power Co., which operates the plant, doubled the number of workers fighting to keep a catastrophe from occurring from 50 to 100. The utility had evacuated 750 workers Tuesday as the remaining 50 struggled to reduce mounting temperatures inside the reactors. The Times said an increasing number of those fighting to prevent a meltdown are military personnel.

The Fukushima plant has six reactors, three of which were shut down when the quake struck. Concerns, however, remained about the pools holding spent fuel rods even at the shut down reactors.

The Japan Times reported a request was issued to police for a water cannon to pump water into the facility. Leaking radiation was measured at 1 rem an hour at midmorning Tuesday, down from 40 rems -- some 400 times the level to which people can be exposed safely in a year. By late afternoon, radiation was measured at 0.15 rem.

Kyodo News reported an unmanned U.S. spy plane was expected to fly over the plant site Thursday.

Flames were reported at the No. 4 reactor Wednesday morning but they died down 30 minutes later. The extent of damage from the fire was not immediately known.

The government ordered injection of Pacific Ocean water to cool down the exposed rods "to avert a major nuclear disaster," Kyodo said.

"'The possibility of recriticality is not zero," the utility said Wednesday referring to the danger of the exposed fuel rods releasing more radioactivity.

To compound the problems at Fukushima, 70 percent of the nuclear fuel rods were estimated to have been damaged at the No. 1 reactor and 33 percent at the No. 2 reactor.

One scientist expressed deep concern to CNN about the workers remaining at the plant.

"It's pretty clear that they will be getting very high doses of radiation. There's certainly the potential for lethal doses of radiation. They know it, and I think you have to call these people heroes," said David Brenner, director of the Center for Radiological Research at Columbia University.

Source: UPI