Activity has been observed at a North Korean nuclear site that is consistent with an effort to restart a reactor, International Atomic Energy Agency chief Yukiya Amano said on Thursday.

North Korea announced in April that it would revive its aged research reactor at the Yongbyon nuclear complex - which experts say is capable of producing plutonium for bombs - but said it was seeking a deterrent capacity.

Amano said the Vienna-based IAEA continued to monitor developments at Yongbyon, mainly through satellite imagery.

"Activities have been observed at the site that are consistent with an effort to restart the 5MW(e) reactor," Amano told the IAEA's 35-nation board, referring to the research reactor.

"However, as the agency has no access to the site, it is not possible for us to conclusively determine whether the reactor has been re-started," he said, according to a copy of his speech.

The Yongbyon reactor has been technically out of operation for years. North Korea destroyed its cooling tower in 2008 as a confidence-building step in talks with South Korea, China, the United States, Japan and Russia.

When North Korea said it planned to revive the reactor, nuclear experts said it would probably take about half a year to get it up and running, if it had not suffered significant damage from neglect.

In September, a U.S. research institute and a U.S. official said satellite imagery suggested North Korea had restarted the facility.

The U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies said a satellite image from Aug. 31 showed white steam rising from a building near the hall that houses the reactor's steam turbines and electric generators.

The reclusive Asian state has defied international warnings not to build atomic bombs and long-range missiles. It is believed to have enough fissile material to build up to 10 nuclear bombs, but most intelligence analysis says it has yet to master the technology to deploy such weapons.

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North Korea said in July it would not give up its nuclear "deterrent" until Washington ends its "hostile policy" towards it, although it was ready to revive nuclear talks.

"I remain seriously concerned" about North Korea's nuclear programme, Amano said. (Editing by Janet Lawrence)