TOKYO — Workers at Japan's earthquake-damaged Fukushima nuclear plant were evacuated early today after radiation levels rose, Chief Cabinet Secretary Yukio Edano said.

TOKYO — Workers at Japan's earthquake-damaged Fukushima nuclear plant were evacuated early today after radiation levels rose, Chief Cabinet Secretary Yukio Edano said.

"All the workers there have suspended their operations. We have urged them to evacuate, and they have," he said, according to a translation by broadcaster NHK.

The news came as white fumes were seen billowing from the plant's No. 3 reactor, with the facility's embattled operator, Tokyo Electric Power Co., or TEPCO, saying the clouds might be steam.

TEPCO said it was unable to check the smoke-like clouds, which began about 10 a.m. local time, as the radiation levels were too high.

Edano said there had been no reports of any explosions at the No. 3 reactor before the fumes were spotted.

NHK, citing nuclear experts, said the clouds could be smoke caused by a fire or a hydrogen explosion caused by the heat inside the reactor.

Edano said Japan may be forced to call on the U.S. military to help cool the reactors, which are located 155 miles northeast of Tokyo.

Engineers have been battling a nuclear emergency at the 40-year-old plant since a 9.0-magnitude earthquake and tsunami — which left more than 3,600 dead and thousands more unaccounted for across Japan — knocked out cooling systems last Friday, leading fuel rods to overheat.

There have been four explosions and two fires at four of the plant's six reactors, and radioactive material has been released into the atmosphere. Two workers have been missing since the disaster struck.

The latest confirmed fire was reported at the plant's No. 4 reactor earlier today, when a worker at the plant saw flames and smoke pouring out of the reactor building's fourth floor about 5:45 a.m. local time, Kyodo News reported, citing information from TEPCO.

The blaze — apparently at the same site as Tuesday morning's fire — was reported to firefighters and, shortly after, seemed to be extinguished.

"We have received information from TEPCO that the fire and smoke is now invisible and it appears to have gone out of its own accord," Minoru Ogoda, a spokesman for the state nuclear safety agency, told AFP.

Radiation levels near the plant had reached levels harmful to health Tuesday, the government said.

After an 18-mile no-fly zone was established around the plant Tuesday, analysts with the US Nuclear Regulatory Commission, or NRC, reviewed data related to the radiation levels in the atmosphere overnight and concluded that the measure was in step with what the U.S. would have suggested in a similar situation.

Eight experts from the NRC were to arrive today to advise on managing the situation.

The hydrogen gas blasts, which began Saturday, have shattered or damaged buildings housing the reactors and a reactor containment vessel may have suffered damage, Edano said.

The government also has reported apparent damage to the suppression pool surrounding the base of the containment vessel of the No. 2 reactor.

The nuclear safety agency, citing information from TEPCO, said 70 percent of the fuel rods at the No. 1 reactor and 33 percent at the No. 2 reactor are believed damaged, judging by radiation levels.

It was possible the rods' metal cladding had melted, exposing the radioactive core, a spokesman said.

Seawater is being pumped around the fuel rods in a desperate attempt to cool them down, but there also are fears about pools which hold spent rods.

If water in the pools evaporates, the spent rods would be exposed to the air and radioactive material would be released into the atmosphere.

TEPCO initially considered spraying boracic acid over the containment pool at reactor No. 4 but now is leaning toward pumping it through fire engines, a nuclear safety agency spokesman said.

Boracic acid curbs nuclear fission by absorbing neutrons, a key element in a nuclear chain reaction.

Scared Tokyo residents filled outbound trains and rushed to shops to stock up on face masks and emergency supplies amid heightening fears of radiation headed their way, AFP reported.