Radiation released by the damaged reactors at Fukushima No. 1 nuclear power plant has been high enough to threat human health, said the Japanese government on Tuesday.

An explosion was heard early Tuesday morning at the No. 2 reactor and another hydrogen blast broke out at the building housing the No. 4 reactor of the nuke plant, which was crippled by the devastating magnitude 9.0 earthquake on Friday.

Radiation near nuclear plant shot up following the explosions and small amount of radioactive substances was later detected in Tokyo, about 250 km from the Fukushima plant.

Radiation amount in Chiba Prefecture was detected twice to four times normal level and radiation in Utsunomiya, Tochigi, was measured 33 times normal level, according to Kyodo news agency on Tuesday.

So far, tens of thousands of people have already been evacuated from an area within a radius of 20 km of the troubled plant. Prime Minister Naoto Kan urged people living within a radius of 30 kilometers of the plant to stay indoors, warning of further leaks.