North Korea, after halting the disassembly of a key nuclear center, is now putting the facility back together in violation of the United States' conditions for improved diplomatic relations between the countries, U.S. officials told FOX News on Tuesday.

The motive isn't clear but sources say North Koreans likely are reassembling nuclear facilities at Yongbyon partly to protest the United States' delay in taking the country off its list of terror-sponsoring nations.

"They've been threatening this move for some time," one U.S. official told FOX News, adding that until now the threats were seen as merely a way for North Korean officials "to express their anger."

Japan's public broadcaster NHK and Kyodo News agency also reported Wednesday North Korea has begun putting its Yongbyon facility back together, days after it halted disablement work.

Even now, piecing the facility back together is seen as a "symbolic gesture" because so much already has been taken apart, though the United States is taking the developments seriously.

Another U.S. official told FOX News that North Korea's intent might be "to put further pressure on us." The cooling tower is gone but the reactor could be back in operation in two to three months, the official said.

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.

Last week, North Korea's Foreign Ministry announced it has stopped disabling the center because the United States failed to remove the communist nation from Washington's terrorism blacklist.

It was the first time that the North had halted the disablement work, though it had slowed the process in protest against the delayed provision of promised aid from its negotiating partners.

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.

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