By Carol Giacomo, Diplomatic Correspondent

WASHINGTON, June 21 (Reuters) - The United States is insisting that multiple facilities at North Korea's Yongbyon complex be shut down, American officials said on Thursday, and these demands could pose new pitfalls for the Feb. 13 nuclear agreement.

The nuclear accord, whose implementation has been delayed for months but has gained significant momentum in recent days, stipulates that Pyongyang will "shut down and seal for the purpose of eventual abandonment the Yongbyon nuclear facility, including the reprocessing facility."

But the accord -- which aims to end a nuclear weapons program that included North Korea's first atomic test last Oct. 9 -- leaves much open to interpretation and further haggling.

Asked how much of Yongbyon the United States wanted shuttered, State Department spokesman Sean McCormack told reporters: "I believe that we have taken an expansive view of what constitutes the Yongbyon facility. .... I think our view is that it encompasses all the buildings ... within that facility."

Jon Wolfsthal, a former on-site monitor at Yongbyon for the U.S. Department of Energy, said the complex has over 100 buildings, including dozens of sensitive facilities.

He told Reuters the administration probably would not insist on shutting every lab and research office.

More likely, the focus would be on the main nuclear reactor, a smaller research reactor, a reprocessing facility, a nuclear fuel fabrication plant, spent fuel storage facility and facilities where radiological materials can be manipulated remotely, said Wolfsthal, now with the Center for Strategic and International Studies.

He said the fact that all the Yongbyon sites to be covered by the shutdown are not detailed demonstrates the "inherent challenge of the February agreement where almost nothing is precisely (spelled out) and every step is going to have to be negotiated and hammered out with the North Koreans."

The chief U.S. negotiator with North Korea, Assistant Secretary of State Chris Hill, made a surprise trip to Pyongyang on Thursday -- the most senior U.S. official to do so in five years -- and was expected to be discussing details of this and other issues.

In February, North Korea committed itself to shut down its main nuclear fuel processing facility at Yongbyon by mid-April after the United States promised to free $25 million in allegedly illicit North Korean funds held in a Macau-based bank.

But freeing the funds took much longer than anyone expected because of political, financial and legal complications.

Wolfsthal said he would be surprised if Pyongyang balked at freezing operations at the main reactor and fuel fabrication facility because once it receives the $25 million it will want to demonstrate good will.

But it could balk at shutting other installations and drive a hard bargain to have them included, he added.

Another senior official, speaking on condition of anonymity, said U.N. inspectors must not only be given physical access to each facility but also to records of the North's nuclear program.

North Korea said on Thursday that a planned visit by U.N. inspectors -- charged by the international community with monitoring and confirming the shutdown of Yongbyon -- was on hold because it had not yet received the $25 million in bank funds, that had been the subject of a lengthy dispute.

But McCormack said he had not heard official confirmation of this new snag.

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