The team led by State Department official Sung Kim discussed the contents of the North's promised nuclear declaration, an issue which is blocking progress on the six-nation pact.

"The negotiations proceeded in a sincere and constructive manner and progress was made there," a North Korean foreign ministry spokesman was quoted as saying by the communist state's Korean Central News Agency.

"I will just tell you that we had a good visit. We had a very substantive discussion," Kim told reporters in South Korea after his return Thursday.

On arrival in Seoul Monday en route to Pyongyang, Kim had said his team would discuss ways to verify the information in the declaration as well as the document itself.

"Everything is subject to verification. That is what we need to focus on," he said.

In last year's landmark agreement, the United States, South Korea, China, Japan and Russia agreed to grant North Korea energy aid and major diplomatic and security benefits in return for full denuclearisation.

But the talks have stalled over the declaration, which was promised by the end of 2007. The North said it delivered the documentation last November but the US described it as incomplete.

In particular, Washington said Pyongyang must clear up suspicions about an alleged secret uranium enrichment programme and suspected proliferation. The North denies both activities.

According to numerous reports, the North in a face-saving gesture will merely "acknowledge" US concerns about the two issues in a confidential document to the United States.

It would detail its admitted plutonium operation, which is based on the Yongbyon reactor, in a formal declaration to talks host China.

Despite hopes of progress, US intelligence officials in Washington were scheduled to tell lawmakers later Thursday that Pyongyang had shared nuclear technology with Syria.

The US has concluded that the North helped Syria begin construction of a nuclear reactor, and that the aid did not stop when the site was destroyed, said one official on condition of anonymity.

Israeli aircraft last September attacked a mystery target widely reported to have been a fledgling nuclear site.

The New York Times and The Washington Post said a video showing North Koreans inside the Syrian reactor, which appears identical in design to one at Yongbyon, would also be shown to lawmakers.

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