The main U.S. nuclear envoy invited his North Korean counterpart to a one-on-one meeting in China amid a prolonged deadlock in arms negotiations, but the North didn't respond to the offer, South Korean officials said Wednesday.

U.S. Assistant Secretary of State Christopher Hill sent a "message" to the North saying he could meet North Korean Vice Foreign Minister Kim Kye Gwan during a trip to China last week, a Foreign Ministry official said on condition of anonymity, citing ministry policy. The official didn't offer any more details.

The unusual offer for direct talks came despite Washington's public insistence it won't meet directly with the North, but only speak to the country with other partners.

It also came amid concern that North Korea may be preparing to conduct a nuclear test, possibly aimed at nudging the U.S. into making concessions. South Korea's main spy agency has said that its northern neighbor could test a nuclear bomb at any time.

Hill and Kim represent their respective countries at six-nation talks aimed at ending North Korea's nuclear program. The negotiating process has been stalled since November because the North refuses to attend in anger over U.S. measures to cut off the North's access to international banking for its alleged illegal financial activity.

South Korea's Vice Foreign Minister Yu Myung-hwan also confirmed Hill made an offer for direct talks.

"I understand that Assistant Secretary Hill made such a gesture on his own initiative," Yu told a regular news briefing, explaining that Hill hoped to revive the six-party talks.

Hill's offer could be seen as an attempt to determine whether North Korea has any willingness to return to the long-stalled nuclear talks.

A South Korean official said Tuesday that Washington is now moving to impose sanctions on the North under a U.N. resolution adopted after its missile tests in July. It was unclear if the North's rebuff of Hill's proposal was connected to the U.S. push.

Hill visited Beijing and other Chinese cities last week as part of an Asian tour that also took him to Japan early last week and South Korea this week.

His unusually long stay in China had aroused speculation about the purpose of the trip, but Hill said the tour was aimed at visiting U.S. consulates across the country.

In April, Hill refused to meet privately with the North's Kim in Tokyo, where all chief delegates to the six-nation talks had gathered for a private security conference, citing the North's refusal to return to the table.

Yu, the South's vice minister, said Hill's recently proposed meeting would have been held under the framework of the multilateral nuclear talks. He didn't elaborate.

Hill has met directly with Kim before. The two met in July 2005 in Beijing to negotiate the North's return to the nuclear talks, ending a previous boycott.

The two countries have also maintained communications through the North's mission to the United Nations in New York. Washington says the channel is only for communication purposes, not negotiation.

In March, U.S. and North Korean officials met in New York to discuss U.S. financial restrictions imposed on the communist nation for alleged illegal activity, including counterfeiting and money laundering.

The nuclear talks -- among the two Koreas, China, Japan, Russia and the U.S. -- were last held in November, when negotiators failed to make progress on implementing a September agreement in which the North pledged to give up its nuclear programs in exchange for aid and security guarantees.