North Korea reactor situation not 'clear': U.N. nuclear chief VIENNA (Reuters) - The U.N. nuclear watchdog is aware of reports that North Korea may have restarted a reactor capable of producing plutonium for weapons, but does not yet have a "clear understanding" of the situation there, its chief said on Thursday. A U.S. research institute and a U.S. official said on Wednesday that satellite imagery suggested North Korea had restarted a research reactor at its Yongbyon nuclear complex. "As we don't have inspectors there, we don't have anything for sure," Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), told reporters in Vienna. The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said a satellite image from August 31 showed white steam rising from a building near the hall that houses the plutonium production reactor's steam turbines and electric generators. Pyongyang announced in April that it would revive the aged Yongbyon research reactor, which yields bomb-grade plutonium, saying it was seeking a deterrent capacity. The U.S. ambassador to the IAEA, Joseph Macmanus, said the IAEA board of governors this week "reiterated overwhelmingly" that North Korea must cease all nuclear activities immediately and refrain from taking any steps to restart its nuclear facilities at Yongbyon. Amano, attending a meeting of the IAEA's 35-nation board, declined to say whether the IAEA, which follows the North's nuclear program via satellite images, had seen any steam coming from the site. Asked whether it was a worrying development, he said: "As we don't have a clear understanding (of the situation), we cannot make a comment." IAEA inspectors have not been allowed into North Korea to undertake verification of its nuclear activities, including the research reactor there, since April 2009, agency spokeswoman Gill Tudor said. "The agency continues its monitoring of the (North's) nuclear activities by available means, such as satellite imagery analysis," Tudor said in an e-mail. Reporting by Fredrik Dahl; Editing by Kevin Liffey