Three workers were exposed to high radiation, two of them are hospitalised. Another strong earthquake hit northeast part of Japan. Technicians restored power to the reactor number one control room of Japan's Fukushima nuclear plant Thursday, even as white steam wafted from four reactors at the tsunami-damaged seaside facility.

This means workers can now use two crucial control rooms, at reactors one and three, which they were earlier forced to abandon after a series of explosions and amid strong radiation and in darkness.

"The light went on in the control room of the number one reactor at 11.30 a.m. (local time), but we are still unsure whether this means the cooling system will be restored," a nuclear safety agency official told AFP.

White steam was seen wafting from reactors one to four, but the causes remained unknown. Officials said no rise in radioactivity had been detected.

Another strong earthquake hit Japan Thursday. A 6.1-magnitude quake hit northeast part of the country, Reuters reported.

After a devastating earthquake and a tsunami that shook Japan on March 11, the country was shaken by numbers of aftershocks.

Three workers at Japan's stricken Fukushima Daiichi plant were exposed to high radiation as they sought to restore power to reactor three, with two hospitalised, the nuclear safety agency said Thursday.