A top US envoy has arrived in the Chinese capital, Beijing, to discuss reviving stalled negotiations on North Korea's nuclear programme.
Mr Hill says North Korea has nothing to gain from boycotting talks
US Assistant Secretary of State Christopher Hill has accused Pyongyang of boycotting multilateral talks on its nuclear ambitions.
He said he was unsure of reports that North Korean leader Kim Jong-il was planning a simultaneous visit to China.
Tensions have remained high since the North's missile tests in early July.
South Korea's Yonhap news agency quotes an anonymous official as saying China had decided to invite Mr Kim amid speculation North Korea may be planning an underground nuclear test.
Pyongyang revealed in February 2005 that it has nuclear capabilities but it has not yet tested a nuclear bomb.
Local media have said a special armoured train, used by the Mr Kim to travel long distances, has arrived at the border with China, prompting speculation that he is about to visit the country.
Asked about the issue, Mr Hill said he had "no information on Kim Jong-il's travels".
"As you know, I take planes and he takes trains," he said. "So I'm not sure, really, what he's doing."
Following talks with his Japanese counterpart Kenichiro Sade, Christopher Hill said both sides had agreed to work with the other partners to revive talks with North Korea.
Some reports suggest the North may be planning more tests
Advocating "concrete action" to persuade Pyongyang to abandon its nuclear programme, Mr Hill said: "The time for organized, multilateral diplomacy is now."
"The problem we've had is that we do not have a negotiator on the other side," he said.
Mr Hill is due to meet officials in Beijing, followed by talks in Chengdu, Guangzhou and Shanghai. He will then move on to Seoul, a US embassy statement said.
Mr Hill maintains that the US position on six-nation talks with Pyongyang, deadlocked since September 2005, had not changed.
"We have no new proposals. We have no incentives, packages, or anything like that," he told journalists in Tokyo on Monday.
"All we have is a September agreement now unfortunately one full year old."
The agreement, which promised economic aid in return for Pyongyang scrapping its nuclear programme, fell apart over disagreements on how to implement it.
North Korea provoked international concern when it launched seven missiles, including a new long-range weapon capable of hitting parts of the US, on 5 July.