The UN atomic watchdog expressed "great concern" Thursday over North Korea and also turned up the heat on Syria and Iran, which are both under investigation for alleged illicit nuclear activity.

International Atomic Energy Agency chief Yukiya Amano told a meeting of the body's board of governors that he was worried by reports North Korea has built a state-of-the-art uranium enrichment facility, which the United States suggested dated much further back than the reclusive Stalinist state claims.

"It was with great concern that I learned of recent reports about a new uranium enrichment facility, as well as the construction of a new light water reactor, in the DPRK" or Democratic People's Republic of Korea, Amano said in his opening address to the closed-door meeting.

Earlier this month a US scientist revealed he had been shown a new uranium enrichment plant at the Yongbyon nuclear complex outside the capital North Korean capital Pyongyang.

The news has heightened international concern that North Korea -- which has conducted two nuclear weapons tests -- could produce highly-enriched weapons-grade uranium on top of the plutonium already in its possession.

The IAEA is not in a position to verify the reports since its inspectors have been barred from North Korea since last year.

Washington's envoy to the IAEA, Glyn Davies, described the revelations as "disturbing" and said the US believed North Korea "has been pursuing enrichment for an extended period of time -- long before April 2009 when the DPRK claims to have begun its Yongbyon enrichment facility construction."

Furthermore, "there is a clear likelihood that DPRK has built other uranium enrichment-related activities in its territory," Davies said.

IAEA chief Amano also appeared to ramp up pressure on Syria, saying he had written to the government there for the first time to try to bring some movement into a stymied two-year-long IAEA probe.

"I wrote a letter to the minister for foreign affairs of the Syrian Arab Republic on November 18 to request the government to provide the agency with prompt access to relevant information and locations" connected to an alleged nuclear site, Amano said.

"I also requested Syria's cooperation regarding the agency's verification activities in general."

It was the first time that Amano has contacted the Syrian government directly with regard to the agency's probe and diplomats close to the IAEA saw it as a sign of his growing impatience with Damascus.

"He's trying to move things along," one diplomat said, speaking on condition of anonymity.

Syria is accused of building an undeclared reactor at a remote desert site called Dair Alzour until it was bombed by Israeli planes in September 2007.

Turning to Iran, Amano complained the Islamic republic was continuing to stonewall a separate investigation there, even as he welcomed the resumption of long-stalled talks between Tehran and world powers.

Iran "has not provided the necessary cooperation to permit the agency to confirm that all (its) nuclear material is in peaceful activities," he said.

Iran was defying UN Security Council resolutions and pressing ahead with its sensitive uranium enrichment activities.

It was also refusing to answer questions about possible military dimensions to its atomic work.

The IAEA has been investigating Iran's nuclear programme for eight years now to try to establish whether it is entirely peaceful as Tehran claims or whether it masks a covert drive to build a bomb as western powers believe.

Iran is under four sets of UN sanctions over its refusal to suspend enrichment of uranium, which can be used to make nuclear fuel or, in highly refined form, the fissile core of an atom bomb.

The IAEA meeting is being held just three days before much higher-level talks in Geneva where Iran is to sit down with the so-called P5+1 grouping of Britain, China, France, Russia and the United States plus Germany for the first time in more than a year.

"I would like to welcome the forthcoming meeting scheduled for next week in Geneva," Amano said.