An explosion occurred at the Unit 2 reactor at the earthquake-crippled Fukushima No.1 plant, the UN nuclear watchdog said on Tuesday, citing information from Japanese authorities.

The explosion occurred at around 6:20 a.m. Japanese local time, the International Atomic Energy Agency (IAEA) said in a statement.

Japanese officials also informed the IAEA that the spent fuel storage pond at the Unit 4 reactor of the plant is on fire and radioactivity is being released directly into the atmosphere.

Dose rates of up to 400 millisievert per hour have been reported at the site, the statement said.

The Japanese officials said that there is a possibility that the fire was caused by a hydrogen explosion.

The latest blasts came after previous explosions at its No. 1 and No. 3 reactors, caused by damage from Friday's earthquake and tsunami, which devastated much of north-west Japan.