Details of the visit by staff from the House of Representatives Foreign Affairs Committee were not immediately available but the department said in a statement that they left "earlier this week on a fact-finding visit."

State Department officials briefed the staff members before their travel, but did not accompany the delegation, it said.

Earlier this year, staff from the Senate Foreign Relations Committee visited North Korea, which has no diplomatic relations with Washington.

Pyongyang has agreed in a landmark six-nation deal to abandon its nuclear weapons program in return for a wide range of economic and political rewards.

Others in the deal are China, the United States, South Korea, Russia and Japan.

Under the deal, the North, which staged a nuclear test in October 2006, last year shut down its key plutonium-producing reactor at Yongbyon and began disabling it.

But under that deal, it was also required to provide a full declaration of all its atomic activities by December 31.

It has not yet done so to the satisfaction of the United States.