A top U.S. diplomat says Washington suspects North Korea has built an additional uranium enrichment facility other than the one it claims it has built recently.

U.S. envoy to the International Atomic Energy Agency (IAEA) Glyn Davies said in a speech to the IAEA's 35-nation governing board on Thursday that it was likely Pyongyang had been pursuing enrichment capability long before April 2009, when Pyongyang claims its latest project started.

He said if indeed the North had been pursuing such a capability, there is a clear likelihood that the communist state has built other uranium enrichment-related facilities.

IAEA Director General Yukiya Amano expressed "great concern" about reports of the new uranium enrichment facility and urged the North to return to the Nuclear Nonproliferation Treaty.