Onagava
© EPA/BGNES Japanese Tsunami survivors survey damage in the devastated town of Onagawa, Miyagi prefecture, Japan, 13 March 2011.
A state of emergency has been declared at a Japanese nuclear facility at Onagawa after excessive radiation levels were recorded there following Friday's earthquake, the UN atomic watchdog.

"Japanese authorities have informed the IAEA that the first or lowest state of emergency at the Onagawa nuclear power plant has been reported by Tohoku Electric Power Company," the International Atomic Energy Agency said.

The alert was declared "as a consequence of radioactivity readings exceeding allowed levels in the area surrounding the plant".

"Japanese authorities are investigating the source of radiation," the watchdog said.

According to the authorities, the three reactor units at the Onagawa nuclear power plant "are under control".

Regarding the ageing Fukushima plant, where an explosion occurred on Saturday, the IAEA said that venting of the reactor Unit 3 had started at 9:20am local time in Japan yesterday "through a controlled release of vapour".

The operation was intended to lower pressure inside the reactor containment, the IAEA said.

Following the failure of the high pressure injection system and other attempts to cool the plant, the authorities had first injected water and then sea water into the unit.

"The authorities have informed the IAEA that accumulation of hydrogen is possible," the statement said.

The IAEA said it was continuing to "liaise with the Japanese authorities and is monitoring the situation as it evolves".