VIENNA (Reuters) - The U.N. nuclear watchdog is following 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 when asked about the reports.

He declined to say whether the IAEA, which follows the North's nuclear programme 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."

(Reporting by Fredrik Dahl; Editing by Kevin Liffey)