"The Japanese authorities have informed the IAEA that radioactivity levels at the site boundary of the Onagawa nuclear power plant have returned to normal background levels," the International Atomic Energy Agency said in a statement.
Earlier, excessive levels at Onagawa had led the authorities to report a state of emergency there.
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"The first or lowest state of emergency was reported at the plant earlier after an increased level of radioactivity was detected at the site boundary," the Vienna-based IAEA said.
"Investigations at the site indicate that no emissions of radioactivity have occurred from any of the three units at Onagawa. The current assumption of the Japanese authorities is that the increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant," it said.
The IAEA said it would continue to liaise with the Japanese authorities and was "monitoring the situation as it evolves."
IAEA chief Yukiya Amaon is to hold a news briefing about the current situation at Japan's nuclear power plants at the agency's headquarters.
