The United States and North Korea wrapped up key talks here on Tuesday over the communist state's nuclear disarmament, as the US warned they were running out of time to resolve an impasse.

US envoy Christopher Hill and North Korea's Kim Kye-Gwan -- who are the chief negotiators for six-nation talks on Pyongyang's nuclear programmes -- met at the US embassy in Singapore for about seven hours, a US official said.

Hill was expected to brief journalists later Tuesday before heading to Beijing for consultations with China and other members of the six-party forum.

Ahead of the meeting, Hill said: "I will be discussing the fact that we are kind of running out of time.

"We're not looking for an agreement. I think we're looking to have a consultation on some of the issues that have kept us apart for several months and certainly I will be discussing them."

Kim did not speak to reporters massed outside the embassy compound before heading into Tuesday's negotiations.

Washington has been pushing North Korea to come clean on its entire nuclear programme as a key step in a 2007 six-nation denuclearisation deal that also involves China, Japan, Russia and South Korea.

The 2007 six-party deal grants North Korea -- which tested an atomic weapon in 2006 -- energy aid and major diplomatic and security benefits in return for full denuclearisation.

The current phase of the deal required the North to disable its main plutonium-producing plants and declare all nuclear activities by the end of last year.

The North says it submitted the declaration in November. But the United States says it has not accounted for an alleged secret uranium enrichment programme or for alleged proliferation to Syria.

"We can't afford any further delays here," Hill said late Monday upon his arrival from Indonesia. "We do need to make some progress very soon."

South Korean media reports suggested that Kim might have been ready to hand over a document in Singapore that addresses concerns about the North's alleged secret uranium enrichment programme and cooperation with Syria.

South Korea's Hankyoreh daily said the US had vowed not to make public the so-called "confidential minute" and not to exploit it for political purposes.

Asked on Tuesday what he expected from the Singapore talks, Hill said: "We've got a pretty open agenda... We will see how it goes."

Hill and Kim last met in Geneva in mid-March.

Asked about a Japanese media report that said North Korea did not understand the US position, Hill responded: "No, it is not possible. They know precisely why, what the issues are and they understand that we didn't want to meet unless we could achieve something."

The US negotiator said the aim was to hold another six-party meeting "very soon".

The talks coincide with increasing tensions between Seoul and Pyongyang. Lee Myung-Bak, a conservative who took office as South Korea's new president in February, has angered the North by adopting a tougher line on ties.

"I hope the talks will be a success, paving the way for the settlement of the North Korean nuclear problem," Lee said in Seoul before a cabinet meeting.

"We will make efforts to improve inter-Korean ties but the six-party talks must also be successful."

North Korea's official media on Tuesday continued their rhetoric, labelling Lee a warmongering "traitor."

Last week the North announced it was suspending all dialogue with the South and closing the border to Seoul officials.

In Beijing, Hill said he would meet South Korean, Japanese, and probably Russian officials as well as the Chinese.