"The fact that they were involved in a covert programme with Syria just underscores the gravity of the proliferation risk and...puts even more weight on the requirement to achieve an effective verification regime," Alexander Vershbow said.

The United States last week accused the North of helping Syria build a reactor, injecting a note of uncertainty into six-nation nuclear negotiations.

The North has not responded but in the past has denied proliferation.

Syrian President Bashar al-Assad has also denied that the site destroyed by an Israeli air raid last September was a nuclear reactor under construction.

The White House has said it is still committed to the six-party talks. It has said it hopes its accusations would prompt Pyongyang to be more willing to disclose atomic and proliferation activities.

Vershvow called for "very strict verification and strict compliance" to ensure no further nuclear cooperation between North Korea and a third country.

"We are not going to compromise our standards in getting the necessary commitments and verification measures from North Korea," he told foreign correspondents.

Under a 2007 deal, the North was to have declared all its nuclear programmes by last December 31 in preparation for the final phase of the agreement -- the dismantling of atomic plants and the handover of all nuclear material.

In return for total denuclearisation it would receive energy aid, a lifting of bilateral sanctions, the establishment of diplomatic relations with the US and a formal peace treaty.

The US says the declaration must clear up suspicions about an alleged secret uranium enrichment programme and the suspected proliferation to Syria.

The North denies both activities. Under a reported tentative deal, it 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 fuelled an October 2006 atomic weapons test, in a formal declaration to talks host China.

A US team visited Pyongyang last week to discuss the declaration and ways to verify it, and both sides reported progress.

The US still believes "we can find a way forward and get to the longest-established goal of the full denuclearisation of North Korea," the ambassador said.

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