A new blast occurred at Japan's Fukushima Daiichi nuclear power complex on Monday, after a massive earthquake and tsunami struck the world's third largest economy.

The blast -- the second hydrogen explosion in three days -- sent out a huge cloud of smoke into the air, while triggering fresh radiation concerns.

According to the Tokyo Electric Power Co. (TEPCO), the plant operator, At approximately 11:01am, an explosive sound followed by white smoke occurred at the reactor building of the Unit 3. It was believed to be a hydrogen explosion.

As of 11:44 am, the measured value of radiation dose near MP6 is 20 microsieverts per hour and the radiation level remains stable, TEPCO said.

TEPCO said though the reactor containment vessel remains intact, the status of the plant and the impact of radioactive materials to the outside environment are presently under investigation.

As of 12:00 am, 4 TEPCO employees and 2 workers of related companies have sustained injuries, but all of them are conscious. However, the local government has warned that those still in the 20-km evacuation zone to stay indoors.

TEPCO also warned that the reactor's cooling system has failed leading to the occurrence of specific incident (failure of reactor cooling function) stipulated in Article 15, Clause 1 of the Act on Special Measures Concerning Nuclear Emergency Preparedness.

..as the Reactor Core Isolation Cooling System failed today, it was determined that a specific incident (failure of reactor cooling function) stipulated in article 15, clause 1 has occurred at 1:25 pm today, the agency said in a statement.

On March 11, Japan's Fukushima Daiichi nuclear complex, 240 km north of Tokyo, was rocked by an explosion after Friday's massive earthquake forced reactors to shut down.

Soon, another blast followed in the building around the No 3 reactor at the Fukushima Number One plant.

The 8.9-magnitude earthquake that struck Japan's Pacific Coast on Friday afternoon, triggered off a wave of destruction with a subsequent tsunami, besides fires and landslides.