The North announced Tuesday it had halted work to make the plutonium-producing plants unusable, and would consider rebuilding them, because the United States has failed to remove it from a terrorism blacklist.

Each side accused the other of violating vaguely-worded six-nation nuclear disarmament agreements.

Washington reiterated that before any delisting the North must agree on ways to verify the declaration of its nuclear activities which it made in June.

A senior South Korean official said the North is trying to "take the upper hand in negotiations" but Seoul and Washington have agreed not to over-react.

Speaking on condition of anonymity, he said Seoul would continue providing energy aid under the six-party deal -- involving the two Koreas, the US, Russia, China and Japan -- unless the situation worsens.

Washington would consult with other parties "to see how things play out in the coming weeks," said US State Department spokesman Robert Wood.

The South Korean official said talks host China had already put forward a compromise verification proposal after the North bridled at original US demands, but this too was rejected by Pyongyang last week.

The North tested an atomic weapon in October 2006 before the disarmament deals last year. In November it began disabling the reactor and other plants at Yongbyon and says 80 percent of the work has been completed.

Analysts say it would take at least a year and up to two to restart the reactor, but the North might raise the stakes in the meantime -- possibly with long-range missile launches.

"The North has apparently decided to stick to its guns and wait for the next administration," said Kim Yeon-Chul, head of Seoul's Hankyoreh Peace Research Institute.

"The US government may want to keep the status quo until the end of the November presidential election but the problem is whether (the US and its allies) can keep the situation under control, as North Korea is highly likely to continue raising tensions in coming months.

"The North may resort to its traditional options for brinksmanship such as the firing of long-range missiles," he told AFP.

Yang Moo-Jin, of Seoul's University of North Korean Studies, said leader Kim Jong-Il "seems to have made a strategic decision that he will no longer continue negotiations with the Bush administration."

Robert Einhorn, a former top State Department non-proliferation expert, said the North Koreans had hoped that by threatening to reverse the nuclear disablement process, Washington would take them off the blacklist.

"At this stage, if the North Koreans are not prepared to make specific commitments on verification, I think the process will bog down, probably for the remainder of the Bush administration," Einhorn said.

"I don't think the Bush administration will cave in on this."

However, he added, the North Koreans were also unlikely to resurrect the reactor which was at the end of its lifespan.

Daniel Pinkston, senior analyst with the International Crisis Group, said the situation was quite serious but that the North Koreans had not completely walked away from negotiations.

"They are seeking to expose any divisions among the other five parties on verification," he said, adding it was unclear whether the North is negotiating in good faith or whether it has no intention of denuclearising.

"Why be in a rush, if you are North Korea? If you are in a bargaining game, whatever your intention is, you can string everyone along."

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