Mr Christopher Hill was in consultations with North Korean envoy Kim Kye Gwan, a US embassy spokesman said.

'Yes, they are meeting at the US embassy,' he said.

The United States and North Korea differ on what was agreed when Mr Hill made a trip to Pyongyang from Oct 1-3 to try to save a shaky February 2007 disarmament deal.

After reaching an apparent agreement on verification procedures, the US announced it would drop the North from a terrorism blacklist, and the North reversed plans to restart its plutonium-producing nuclear plants.

However, North Korea, which tested an atomic weapon in October 2006, insists it never agreed to samples of atomic material being taken away.

It says the outside verification of its nuclear inventory, submitted in June, will involve only field visits, confirmation of documents and interviews with technicians.

'When we get to verification we want to have a clear roadmap of how verification is being done,' Mr Hill told reporters earlier on Thursday after arriving from Tokyo.

'We don't want a situation where, when we get to verification, there is some misunderstanding on what we agreed to.'

A man who answered the phone at North Korea's embassy in the early afternoon said nobody could immediately comment on the talks.

US Secretary of State Condoleezza Rice said a meeting in Beijing between all six nations involved in talks on the North's nuclear programme aims to finalise a plan allowing for outside verification of Pyongyang's disarmament.

Dr Rice said the meeting would take place on Monday, although host Beijing has yet to announce a date for the talks.

'China seems to be waiting for North Korea's final answer after the Singapore talks,' South Korea's Yonhap news agency on Wednesday quoted a diplomatic source as saying.

Mr Hill, who is to leave Singapore on Saturday, earlier met in Tokyo with his Japanese counterpart Akitaka Saiki and South Korean nuclear envoy Kim Sook, who agreed to try to get Pyongyang to commit on paper to a framework for verifying its nuclear disarmament, officials said.

The documents should cover the removal of samples from North Korea's nuclear facilities, despite Pyongyang's strong objections, they agreed.

'There should be no room for misunderstanding or distortion when we actually begin verification,' Mr Saiki told a news conference.

'The six parties must agree to specify exactly what we are supposed to do in the form of documents,' he said. 'We have agreed on this basic point.'

The US State Department has reiterated that sampling is part of the deal which Mr Hill reached in October, although it would not say whether there is a written agreement.

United States officials say sampling is crucial to checking how much bomb-making plutonium the North produced in the past - and how many bombs it could theoretically make from its stockpile.

Mr Hill and Mr Kim held an earlier round of talks in Singapore last April.

The choice of Singapore as a venue reflects Pyongyang's strategy to put the nuclear issue on a bilateral footing with Washington, said officials in Seoul who were cited by Yonhap.

'North Korea has an embassy in Singapore and it also provides the North Korean delegation with various services including financial support,' Yonhap quoted a South Korean foreign ministry official as saying.

Singapore and North Korea have had diplomatic relations since 1975. -- AFP