South Korea's nuclear envoy flew to the United States Thursday for talks on North Korea amid efforts by China to restart long-stalled negotiations on Pyongyang's nuclear disarmament.

Wi Sung-Lac will meet Deputy Secretary of State James Steinberg, the US nuclear envoy Sung Kim and Stephen Bosworth, special representative for North Korea policy, "to discuss overall issues related to the six-party talks," a foreign ministry spokesman said.

Wi's three-day trip comes after China's nuclear negotiator Wu Dawei had a flurry of meetings with counterparts in South Korea, Japan and the United States following a visit to Pyongyang last month.

In North Korea, Wu discussed a possible resumption of the six-party disarmament talks deadlocked since the last meeting in December 2008.

The talks hosted by China also group the two Koreas, Japan, Russia and the United States.

The North walked out of the forum in April 2009 and staged its second atomic weapons test a month later, triggering a series of sanctions from the UN Security Council and more recently the United States.

During a visit to Seoul last month, Wu said Pyongyang supports Beijing's proposal to hold informal preliminary meetings before resuming the full-fledged negotiations.

But Seoul has expressed reservations about China's proposal, with Foreign Minister Yu Myung-Hwan saying the North should first take action to show its seriousness about disarmament.

The North's alleged attack on a South Korean warship in March that killed 46 sailors further complicated the process as inter-Korean tensions rose sharply.

Seoul has refused to offer much-needed food aid to the North devastated by recent floods, despite its snowballing rice reserves.

Vice Unification Minister Um Jong-Sik said rice aid needs "a more careful approach."

"When the six-party talks can resume really depends on the North's attitude... the North first needs to show a responsible attitude about the Cheonan ship sinking incident and show its sincerity about denuclearisation through actions," he said in a radio interview Thursday.

North Korean leader Kim Jong-Il reportedly told President Hu Jintao during a visit to China last week that his country is willing to return to the nuclear forum.

China's Wu met Steinberg in Washington Wednesday. "Six-party talks are an important way to maintain peace and stability in the Korean Peninsula," he said afterwards.

The Chinese "shared their perspective on where we are, their interpretation of what needs to be done based on their high-level conversations with North Korean leaders", said State Department spokesman Philip Crowley.

"We have our own ideas. We'll be consulting further."