"The wind direction for the time being seems to point the (nuclear) pollution towards the Pacific," Andre-Claude Lacoste told journalists after the blast at the Fukushima No. 1 plant in the north of the country.

The explosion at the ageing plant raised fears of a possible meltdown a day after the facility's cooling system was damaged in Japan's massive earthquake on Friday.

"Apparently the situation is serious," Lacoste said, adding that his team was receiving incomplete information from Japan because of the number of people tied up with managing the crisis.

The Japanese government has declared an atomic emergency and said tens of thousands of people living within 20 kilometres of the plant should leave.

But the operator Tokyo Electric Power (Tepco) said the structure encasing the reactor had collapsed during an aftershock but the steel reactor inside it was not ruptured.