SEOUL, South Korea — North Korea said Tuesday it has suspended work to disable its nuclear reactor in anger over Washington's failure to remove it from the U.S. list of terror sponsors. The North said it will soon consider a step to restore the plutonium-producing facility.

The announcement poses the biggest hurdle yet to the communist nation's denuclearization process under a landmark deal last year.

"The U.S. postponed the process of delisting the (North) as a 'state sponsor of terrorism,"' the Foreign Ministry said in a statement carried by the official Korean Central News Agency. "Now that the U.S. breached the agreed points, the (North) is compelled to take" countermeasures, it said.

The Foreign Ministry also said the government will "consider soon a step to restore" the nuclear facility at Yongbyon, but it did not elaborate. The disablement was suspended as of Aug. 14, it added.

U.S. Secretary of State Condoleezza Rice said North Korea still has obligations to fulfill.

"The work ahead is to get that verification mechanism and therefore to proceed with denuclearization," she said during a news conference in Ramallah, West Bank. "We actually are in discussions with the North Koreans and I think we'll just see where we come out in a few weeks."

The U.S. offered to remove North Korea from the list of state sponsors of terrorism as one of the key concessions in exchange for the North shutting down and disabling the nuclear reactor under a deal reached last year in six-party negotiations that include China, Japan, the two Koreas, the U.S. and Russia.

In June, the U.S. said it would remove North Korea from the list after it turned in a long-delayed account of its nuclear programs and blew up the reactor's cooling tower in a symbolic move to demonstrate its commitment to disarm.

North Korea began disabling the plutonium-producing facilities in November but the North slowed the work in a dispute with Washington over how to verify a declaration of its nuclear programs.

The two sides have been negotiating on that issue with Washington insisting it would remove the North from the terror list only after it agrees to a verification plan.

That has angered North Korea.

The North's statement came shortly after Chinese President Hu Jintao left South Korea where he held talks with President Lee Myung-bak on the North Korean nuclear issue among other topics.

South Korean and Japanese officials lamented the North's move.

"It's regrettable that this announcement came at a time when each side has been trying" to move the process forward, said Kim Sook, Seoul's chief nuclear envoy. "I hope North Korea will resume disablement measures at an early date."

Japan said it was worried by the development.

"We take it with grave concern," Japanese Foreign Ministry spokesman Kazuo Kodama told a news conference in Tokyo.

The North's state media have issued a series of commentaries blasting the U.S. and the Foreign Ministry last week threatened that the country would bolster its "war deterrent" — a euphemism for its nuclear programs — as it condemned U.S.-South Korea military exercises.

Last week, the state news agency KCNA also lashed out at Bush, accusing him of blocking progress at the nuclear talks by raising the issue of human rights in the North. During a trip to Seoul earlier this month, Bush publicly criticized the North's human rights record.

Analysts were divided over the North's ultimate aims.

Yang Moo-jin, a professor at Seoul's University of North Korean Studies, called Tuesday's statement very serious.

"I think this represents the biggest crisis to the denuclearization process since the Feb. 13 agreement," Yang said, referring to last year's disarmament-for-aid deal. "The North's Kim Jong Il may have decided that he won't negotiate with the Bush administration any more" unless Washington takes the North off the terror list first, he said.

Bush is set to leave office in January after U.S. presidential elections in November.

But Cheong Seong-chang, a North Korea expert at the private Sejong Institute in South Korea, said the latest statement appeared to be aimed at pressuring Washington to lower its demands regarding verification and remove the North from the terror list.

South Korean and U.S. officials have said eight of the 11 disablement measures have been finished and that when the entire disablement is completed, it would take at least a year for the North to restart the facilities.

Whang Joo-ho, a nuclear expert at South Korea's Kyung Hee University, said it would take about three to six months for North Korea to restore its nuclear facilities at Yongbyon. He said it would take only one month to rebuild the kind of cooling tower the North destroyed in June.

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Associated Press writers Kwang-tae Kim and Hyung-jin Kim in Seoul and Shino Yuasa in Tokyo contributed to this report.