North Korea said Tuesday it has stopped disabling its nuclear reactor and will consider restoring the plutonium-producing facility in anger over Washington's failure to remove it from the US list of terror sponsors. The North's announcement marks the emergence of the biggest hurdle yet to the communist nation's denuclearization process under a landmark deal last year and is expected to escalate tension in the nuclear talks involving China, Japan, the two Koreas, the US and Russia. Pyongyang's Foreign Ministry said it suspended the disablement work at the reactor and other facilities at its Yongbyon nuclear complex as of August 14 because the US did not keep its promise to delist Pyongyang as a terror sponsor under last year's deal. The countries concerned were notified of the suspension, the ministry said. "The US postponed the process of delisting the (North) as a 'state sponsor of terrorism,"' the ministry said in a statement carried by the official Korean Central News Agency. "Now that the US breached the agreed points, the (North) is compelled to take" countermeasures, it said. The ministry also said the country will "consider soon a step to restore" the Yongbyon nuclear facilities, but did not elaborate. The statement came shortly after Chinese President Hu Jintao left South Korea after summit talks with President Lee Myung-bak that included discussions on the North Korean nuclear issue. South Korean 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." Seoul's Foreign Ministry issued a statement containing similar remarks. Removal from the terror list is one of the key concessions offered to the North in exchange for shutting down and disabling the reactor under a landmark six-nation deal reached last year. In late June, the US announced that it would delist the North as a terror sponsor after Pyongyang 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 denuclearization commitment. The two sides have been negotiating how to verify the nuclear declaration, with Washington insisting it would remove the North from the terror list only after Pyongyang agrees to a verification plan. That has angered Pyongyang. The North's state media have issued a series of commentaries blasting the US, and the Foreign Ministry last week threatened that the country would bolster its "war deterrent" - a euphemism for its nuclear programs - as it condemned US-South Korea military exercises. Last week, KCNA, the news agency, 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 Pyongyang'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" and said that it could mean Pyongyang may have decided not to deal with the administration of US President George W. Bush. "I think this represents the biggest crisis to the denuclearization process since the February 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 Pyongyang off the terror list first, he said. Bush is set to leave office in January next year. The U.S. will elect a new president 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. North Korea began disabling the plutonium-producing facilities in November, but the process had been delayed because Pyongyang slowed the work in a row with Washington over how to declare the nuclear programs. South Korean and US 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.