North Korea has crossed a new threshold in rebuilding a prominent nuclear facility, breaking seals placed by the International Atomic Energy Agency, FOX News has learned.

Two senior U.S. officials familiar with the intelligence told FOX News that North Korea has stepped up its work putting the Yongbyon facility back together in violation of the United States' conditions for improved diplomatic relations between the countries.

"The North Koreans are definitely reassembling the plant," one official said. "The IAEA seals are not proving to be an impediment. In fact, they are breaking them, adding pipes, valves, etc., and then replacing the seals.

FOX News has also learned that the equipment that was removed from the reprocessing facility, one of three facilities at Yongbyon, was refurbished rather than destroyed. So the actions the North Koreans had taken so far in disabling their nuclear plant were in essence "reversible disablement," the U.S. officials said.

The exact motive isn't clear, but the North Koreans likely are protesting the United States' delay in taking the country off its list of terror-sponsoring nations.

It may not be time to panic, but U.S. officials are concerned that further steps could put the North Koreans within six to eight weeks of producing plutonium.

North Korea began disabling its plutonium-producing nuclear reactor and other facilities at its Yongbyon nuclear center in November as a step toward their ultimate dismantlement in exchange for economic aid and political concessions, including removal from the U.S. terror list.

The United States announced in June that it would take North Korea off the terror list after Pyongyang turned in a long-delayed account of its nuclear programs and blew up the cooling tower at the reactor.

The two sides have since been negotiating how to verify the nuclear declaration, but no agreement has been reached. Washington has insisted it will remove North Korea from the terror list only after the country agrees fully to a verification plan.

U.S. Assistant Secretary of State Christopher Hill said Friday after meetings in Beijing that the U.S. was willing to sit down again with representatives from the five other countries involved in the disarmament talks.

"What we need to do is verify their nuclear declaration and we have put together a protocol that's based on international standards," Hill told reporters after meeting with South Korean and Japanese envoys. "I think there's a lot of support within the six-party process for getting this done."

But Hill said there were no plans for talks with North Korea before he heads back to Washington.

FOX News' Nina Donaghy, Jennifer Griffin and Justin Fishel and the Associated Press contributed to this report.