TOKYO (AP) -- A U.S. special envoy said Thursday that any move by North Korea to restart a nuclear reactor would be a "very serious matter" and violate United Nations Security Council resolutions.
A U.S. research institute said Wednesday that a recent satellite image appears to show that North Korea is restarting its plutonium reactor at the Nyongbyon nuclear facility, which was shuttered in 2007 under the terms of a six-nation disarmament agreement.
U.S. special envoy for North Korea Glyn Davies told reporters in Tokyo on Thursday that if the report is true, it would be a violation of North Korea's past commitments.
"This would be a very serious matter — we think a misstep on the part of North Korea — because, of course, it would violate a series of U.N. Security Council resolutions," Davies said after meeting with his Japanese counterpart, Junichi Ihara.
"It flies in the face of North Korea's own commitments and promises they've made over the years," he said. "So this would be a step that we would regard very seriously."
North Korea threatened in April to restart the reactor but has since toned down its inflammatory rhetoric and stepped up diplomacy with rival South Korea.
The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said an Aug. 31 satellite image shows white steam rising from a building next to the North Korean reactor and that it appears to indicate that it is being restarted.
South Korea has yet to respond to the report. Its National Intelligence Service said it could not confirm it because confidential information is involved.
Davies earlier visited China to discuss North Korea's nuclear program. China called Thursday for joint efforts to denuclearize the Korean Peninsula.
"To achieve denuclearization and to maintain peace and stability is what China has been advocating," Chinese Foreign Ministry spokesman Hong Lei said at a daily briefing. "It takes efforts from all sides."
Hong said all sides should work to resume six-nation talks on North Korea's disarmament.
Associated Press writer Didi Tang in Beijing contributed to this report.