Radioactivity levels at the site boundary of the Onagawa nuclear power plant in Japan have returned down to normal background levels, the UN nuclear watchdog said on Sunday.

The first (lowest) state of emergency was reported at the plant earlier on Sunday after an increased level of radioactivity was detected at the site boundary, the IAEA said in a statement quoting Japanese officials.

However, investigations at the site indicate that no emissions of radioactivity have occurred from any of the three units at Onagawa, the statement said.

The three reactor units at the power plant are now under control, Japanese officials told the IAEA.

The IAEA said the increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant, where explosion had occurred at its unit 1 reactor.