"Radiation dose rate measurements observed at four locations around the plant's perimeter over a 16-hour period on March 13 were all normal," the International Atomic Energy Agency said in a statement.

An explosion shook the Fukushima nuclear power plant today, injuring six people, but officials said the reactor container was not breached and there was no major rise in radiation.

The blast was caused by a build-up of hydrogen in the building around the No 3 reactor at the Fukushima Number One plant, said chief government spokesman Yukio Edano.

A similar explosion hit the building containing the No 1 reactor at the plant on Saturday, the day after a devastating earthquake and a tsunami struck a large swathe of the country's Pacific coast north of Tokyo.

"Based on information provided by Japanese authorities, the IAEA can confirm the following information about the status of Units 1, 2, 3 and 4 at Fukushima Daini nuclear power plant," the Vienna-based said in its statement today.

"All four units automatically shut down on March 11. All units have off-site power and water levels in all units are stable. Though preparations have been made to do so, there has been no venting to control pressure at any of the plant's units."

At unit 1, plant operators had been able to restore a residual heat remover system, "which is now being used to cool the reactor. Work is in progress to achieve a cold shutdown of the reactor," the IAEA said.

"Workers at units 2 and 4 are working to restore residual heat removal systems. Unit 3 is in a safe, cold shutdown."