The United Nations nuclear agency said it had seen releases of steam and water indicating that North Korea may be seeking to restart a reactor that experts say would be capable of making plutonium for atomic bombs.
North Korea announced in April that it would revive its aged research reactor at the Yongbyon nuclear complex but said it was seeking a deterrent capacity.
Yukiya Amano, director general of the International Atomic Energy Agency, said on Thursday 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," he told the IAEA's 35-nation board, referring to the Yongbyon 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.
He later told a news conference that the IAEA has "very recently seen activities at Yongbyon that point to possible testing" of the reactor, including the release of steam from vents and the apparent discharge of water into a river.
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.
"We are aware that North Korea is test-running the Yongbyon nuclear reactor," said South Korean defence ministry spokesman Wi Young-seop. "And we're keeping an eye on whether the North is running it in earnest."
In September, a US research institute and a US official said satellite imagery suggested North Korea had restarted the facility.
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.
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.