Seoul - US envoy Christopher Hill has extended his stay in North Korea for at least a day, leading to speculation Thursday that there may be progress in his efforts to save a disarmament deal.

South Korea was informed by the US that Hill, who traveled overland Wednesday to Pyongyang, would not return to Seoul on Thursday and had extended his stay until at least Friday, according to a Foreign Ministry official cited by Yonhap News Agency.

There was no reson given for the extension and It was uncertain whether Hill will end his trip to the North on Friday or stay there longer.

On Wednesday, in Washington, the State Department was taking a wait-and-see approach.

"The ball is, in essence, in the North Koreans' court," department spokesman Sean McCormack said at a press briefing. "And we will see if they make a different set of decisions than they've been making over the past month."

US officials in Washington had earlier confirmed that Hill would offer a face-saving compromise on verification in his attempt to rescue the 2007 agreement which led the North to shut down its plutonium-producing plants.

Upon his arrival in Seoul on Tuesday Hill said talks were in "a very difficult, very tough phase."

Washington said it would not fulfil its promise to remove North Korea from its list of state sponsors of terrorism until Pyongyang agrees to procedures to verify the declaration.

North Korea accused the United States of failing to keep its end of the bargain and last week removed seals on its nuclear facilities that were placed there by the UN's international nuclear watchdog and said it would resume work at its plutonium-reprocessing plant.

The plant at its main nuclear facility in Yongbyon, 100 kilometres north of Pyongyang, can be used to produce plutonium, which in turn can be used to make nuclear weapons. (dpa)