The head of the UN atomic watchdog expressed "great concern" Thursday over North Korea as the United States alleged Pyongyang's uranium enrichment activities dated much further back than it claimed.

"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" (Democratic People's Republic of Korea), Yukiya Amano told the board of governors of the International Atomic Energy Agency.

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

The news heightened international concern that the reclusive Stalinist state -- 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.

"To my regret, the agency has not had inspectors in the DPRK since April last year, and the DPRK has not permitted the agency to implement safeguards in the country since December 2002," Amano said.

"I urge the DPRK to fully implement all of the relevant resolutions of the (IAEA) general conference and of the (UN) Security Council. As the only multilateral organisation for nuclear verification, the IAEA has an essential role to play in verifying the DPRK's nuclear programme," he said.

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."

Davies cited the apparent scale of the facility, the speed with which it was built and Pyongyang's past efforts to procure enrichment-related material from abroad as evidence for Washington's belief.

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

The European Union, in its own statement, said it was "greatly concerned" by the revelations and urged Pyongyang "to cease forthwith these construction activities ... and to give IAEA inspectors access to all its nuclear facilities and relevant documentation without further delay."