As Japan is struggling to contain a spiraling nuclear crisis, another fire broke out again at a quake-crippled nuclear plant on Wednesday, raising fears of more radiation contamination.

The fire struck on early Wednesday at the building housing of the No.4 reactor of the Fukushima Daiichi plant, 240 km (150 miles) north of Tokyo, said Tokyo Electric Power Co.(TEPCO), the plant's operator.

Workers saw a blaze erupt at 5:45 a.m.(2110 GMT) at the northwestern corner of the building's fourth floor and about 30 minutes later, the smoke and fire could no longer be seen within 10 meters away, public broadcaster NHK reported.

It is the second fire to hit the spent fuel storage pond at No.4 reactor in just over 24 hours, signalling a deterioration in the situation.

On Tuesday, a fire broke out in the same reactor's fuel storage pond, an area where used nuclear fuel is kept cool, sending radiation into the atmosphere.

Tokyo Electric Power said the new blaze erupted because the initial fire had not been fully extinguished.

Unlike other three reactors that saw explosions over the past few days, the No.4 reactor was shut down for maintenance when the quake and tsunami struck last Friday.

Reactors in the No.5 and No.6 units also saw an increase in temperatures, although there have been no reports of abnormalcies or explosions.

On Wednesday, Japan's nuclear safety agency said 70 percent of the nuclear fuel rods may have been damaged at Fukushima Daiichi No.1 reactor that was first stricken last week, triggering the crisis.

At the moment, detectors showed 400 millisieverts per hour near the No.3 reactor, and 100 millisieverts at No.4 reactor, Chief Cabinet Secretary Yukio Edano said. Exposure to over 100 millisieverts a year is a level which can lead to cancer.

The crisis at the Fukushima Daiichi complex which houses six nuclear reactors began when the historic 9.0-magnitude earthquake devastated northeast Japan and knocked out its cooling systems.

Two hydrogen explosions rocked No.1 reactor and No. 3 reactor on Saturday and Monday respectively due to the failure of the cooling function, damaging the outer walls. But it did not pierce the reactors' steel and concrete containment vessel.

On Tuesday morning, a third blast occurred at the No.2 reactor and damaged the reactor's suppression pool, raising radiation in the area to dangerous levels.

Prime Minister Naoto Kan warned of further leaks and told people living within a radius of 30 km of the troubled plant to stay indoors.

Following Tuesday's explosion, radiation levels in Tokyo, one of the world's most populous cities, rose 10 times above average, spreading fear among many of the 33 million residents in the metropolitan area.

Concern now centers on damage to the No.4 reactor which stores spent rods in pools of water outside the containment area, and also to the No.2 reactor that helps cool and trap the majority of cesium, iodine and strontium in its water.

If the government fails to put the No. 2 reactor under control, cooling work on No.1 and No.3 reactors would be forced to halt, experts warned.

Authorities have been flooding the chambers of the quake-shaken reactors with sea water to cool the reactors down in an effort to avert a possible meltdown.

Prime Minister Kan said earlier Tuesday that his government and TEPCO will set up integrated headquarters, headed by the premier, to address issues at the Fukushima No. 1 plant.

Right now, U.S. and Russian nuclear experts are rushing to Japan to help the country deal with its nuclear safety crisis. On Monday, the Japanese government had appealed to the IAEA for help.