An explosion was heard Saturday at the Fukushima No. 1 nuclear power plant, destroying the walls of the reactor building, local media reported.

Following the blast which supposedly occurred at around 3:36 p. m. local time (0636 GMT), Chief Cabinet Secretary Yukio Edano said at an emergency press conference in Tokyo that people should stay calm and that the radiation levels are being monitored carefully.

Further investigations are underway to discover the severity of the damage, government officials said, but local reports suggested the main building of the plant housing the reactor as well as the outer walls were blown away in the explosion.

Japan's Nuclear and Industrial Safety Agency officials said that they believe there has been no serious damage to the container of the troubled No. 1 reactor of the Fukushima No. 1 nuclear power plant, located about 220 km north of Tokyo.

The Nuclear and Industrial Safety Agency said earlier that a nuclear reactor at the Fukushima No. 1 plant, may be starting to melt down after Japan's biggest earthquake on record hit the area Friday.

TV footage showed smoke billowing from the site and the reactor building had been destroyed.

A total of 45,000 residents living within a 10-km radius of the leaking plant have been told to evacuate their homes by local authorities, with the evacuation zone expected to be widened to 20 kilometers imminently.

Four people have been injured at the power plant, authorities said. But radiation levels dropped quickly after surging for a while, following the blast.

The reports of the explosion followed aftershocks and came amid a mammoth search and rescue mission launched in the northeastern region of Japan involving both Japanese and U.S. military forces.

The Tokyo fire department deployed a specialized nuclear rescue team to the area to assist with the catastrophe, officials said.

As international rescue teams arrive, 1,600 people are missing or dead following Friday's colossal earthquake and the ensuing enormous tsunami.

The latest police tally showed that the death toll has reached 564.

Japan's public broadcaster NHK reported that the Self-Defense Forces (SDF) rescue team found 300 to 400 bodies Saturday in Rikuzetakata, a coastal city of Iwate Prefecture, which had been smashed by huge quake-triggered tsunami.