The chief American negotiator on North Korea's nuclear program expressed frustration Tuesday with Pyongyang's refusal to return to six-nation talks amid concerns the regime might be preparing a nuclear test.

China, meanwhile, denied that arrangements were under way for a visit by the North's reclusive leader, Kim Jong Il, despite media reports that his special train had arrived in a city on the Korean-Chinese border.

The "North Koreans apparently have no interest right now in joining the diplomatic process," U.S. Assistant Secretary of State Christopher Hill told reporters after arriving in Beijing to meet with his Chinese counterpart. "We regard this as not only too bad for the process but too bad, too, for them."

For the second time in as many weeks, South Korean media reported that Kim was in or about to visit China, with the JoongAng Ilbo newspaper saying Tuesday that Kim's visit was likely to begin within the next few days.

However, Chinese Foreign Ministry spokesman Qin Gang said at a regular media briefing that he knew of no such trip to Beijing by the North Korean leader, though one has been rumored for weeks.

CountryWatch: North Korea

South Korea's main spy agency warned last week that Pyongyang could test a nuclear device at any time. The warning followed reports of suspicious activity at a suspected North Korean underground nuclear testing site.

The North claims to have nuclear weapons but has not performed any known tests. To test now would escalate tensions in the region, setting up a confrontation with the U.S. and scuttling the chance for resuming long-stalled negotiations over Pyongyang's nuclear programs. It would also anger China, North Korea's main patron.

Hill said he expected to have in-depth conversations about the North Korean nuclear issue on Wednesday morning when he meets his Chinese counterpart Wu Dawei, China's point-man in the six-nation nuclear talks.

He said the U.S. wanted to consult with the Chinese about the possibility of a North Korean nuclear test or a repeat of the North's missile launches in July.

"I think we will be able to discuss some of those issues," he said. "How can we prevent them from happening and if they happen anyway, what will we then do about that. That's something we do want to be in close contact with the Chinese about."

Hill said he had discussions with Chinese Vice Foreign Minister Cui Tiankai about the six-party process but also about regional issues and the Iranian nuclear issue, but he would not give details.

Hill met with his Japanese counterpart Kenichiro Sasae in Tokyo on Monday. The two agreed to cooperate with China, Russia and South Korea to bring the North back to the talks, which are aimed at persuading the reclusive regime to give up its nuclear ambitions. Pyongyang has boycotted the negotiations until Washington lifts financial restrictions aimed at the North.

Hill said the nations also were open to other configurations, such as the so-called five-plus-five format that brought five of the six-party nations and five other regional nations together for talks in Kuala Lumpur, Malaysia, in July.

South Korea's JoongAng Ilbo quoted an unidentified person in Beijing as saying that Kim's train has been in the northeastern border city of Shinuiju and there is a high possibility of Kim crossing into China in the next few days.

The newspaper cited another person as saying a team of North Korean security officials had visited Beijing on Aug. 25 to prepare security arrangements for Kim's trip.

Meanwhile, South Korea's Yonhap news agency reported that North Korea had blocked off roads leading to Shinuiju, a possible sign of security measures indicating that Kim could be passing through the area. The report cited an unnamed source in the Chinese border city of Dandong, across from Shinuiju.

Kim rarely travels abroad, but has occasionally visited allies China or Russia, last traveling on a tour through several Chinese cities in January. Beijing and Pyongyang didn't officially acknowledge Kim had been in the country until after he left.

Relations between the communist allies were strained after North Korea launched missiles in July and China joined in a U.N. Security Council resolution condemning the tests.