US nuclear envoy Christopher Hill began a visit to North Korea on Wednesday in a bid to break the nuclear deal stalemate with Pyongyang, which threatened to pull out from a 2007 disarmament deal.

North Korea previously told the International Atomic Energy Agency that it would reintroduce nuclear material to its Yongbyon reactor, accusing the United States of reneging on a pledge to strike the country off its list of state sponsors of terrorism.

"We strongly urge the North to reconsider these steps and come back into compliance with its obligations as outlines in the six party agreements," Gordon Johndroe, a White House spokesman, said last week.

South Korea's Yonhap news agency has quoted an unnamed South Korean government source as saying there are signs of activity near the site of North Korea's 2006 underground nuclear test.