North Korea's decision to indefinitely pull out of talks on its nuclear programme appears, on the surface, to have come out of the blue.
By Sarah Buckley
Only last month, Pyongyang said it was ready to treat the US as a "friend", and officials in the region expected new talks to start in a matter of weeks.
The turnaround, according to North Korea, was provoked by recent high-profile speeches by the Bush administration, including Secretary of State Condoleezza Rice's branding of Pyongyang as an "outpost of tyranny".
North Korea last took part in talks on its nuclear programme in June
But analysts said the real reason was last week's briefing of Asian officials by Michael Green, a senior US envoy.
During visits to Japan, South Korea and China, he is believed to have alleged that North Korea sold uranium hexaflouride - which can be used to enrich uranium for nuclear weapons - to Libya in 2001.
If true, that transaction would undermine North Korea's claim that its nuclear arsenal is purely defensive, and threaten to escalate its row with the US to a dangerous new level.
"Nuclear proliferation is the ultimate red line from Washington's point of view," said John Swenson-Wright, associate fellow at The Royal Institute of International Affairs (RIIA) in London.
It is not yet clear if North Korea's announcement was motivated by anger or alarm. Most analysts agree its policy on the nuclear talks had been on hold until the new Bush administration's approach became clearer.
It may be that Mr Green's hardline briefing persuaded Pyongyang that nothing had changed.
But, as always with North Korea, analysts point out that Pyongyang could have withdrawn simply in order to gain leverage in the long-running negotiations.
In withdrawing from the talks "indefinitely", but not abandoning them completely, it may be hoping to win economic incentives from neighbours South Korea and China to coax it back to the negotiating table.
It may also hope to dilute US demands - which include the complete dismantlement of the North's nuclear programmes - although Washington has said that these are non-negotiable.
The US thinks North Korea has been selling enriched uranium
Mr Green's Asian tour may have triggered North Korea's withdrawal, but Pyongyang's frustration has been building for a number of months, said Adam Ward, of the International Institute of Strategic Studies.
North Korea has been under pressure to respond to Washington's proposal, made during talks in the summer, that it follow Libya and disarm.
It is also threatened with sanctions by Japan, because of a row over Japanese it kidnapped to train North Korean spies in the 1970s and 80s.
In addition, Pyongyang has been angered by a US Bill passed last year which aims to promote better human rights in North Korea and protect North Korean defectors.
"The US has been talking about nuclear non-proliferation until the third round [of six-party talks]. Now, the US has put the issue of human rights on top of the issue of non-proliferation," said Jasper Kim, assistant professor of international studies at Ewah Women's University in Seoul.
What happens next
Despite the dramatic nature of Thursday's agreement, analysts suspect North Korea will probably return to the talks eventually.
Part of the reason why this crisis has been dragging on so long is because there are very few options on the table.
Mr Swenson-Wright said North Korea had two alternatives.
It could either ratchet up tensions further, by testing a nuclear weapon, for example, or return to the talks.
The former is "not likely because it would evoke a strong reaction from everyone, including [ally] China", he said.
As for the US and its allies in the region, their options are also limited.
Military action is highly unlikely because of the size of North Korea's conventional army, and the international consensus that it does have a nuclear weapons programme, and at least six to eight nuclear weapons already in its arsenal.
The US is also currently preoccupied with Iraq and Iran so diplomacy is its only choice, analysts said.
But if the talks are to get back on track, North Korea's closest ally, China, holds the key, according to Robyn Lim, professor of International Relations at the University of Nanzan in Nagoya.
China is one of the few countries to have any influence over the North.
"But the basic problem is the Chinese themselves are pretty divided," she said, pointing to splits between the military and civilian administration as to how much pressure to exercise on Pyongyang.
The risk to China of a collapse in the North Korean regime are enormous. Political disintegration could result in floods of refugees over its borders, and a reunification with South Korea could mean greater US influence in the region.
Beijing, like the other parties involved, does not want nuclear weapons on the Korean peninsula.
But the problem is, the longer there is no progress on talks, the more time North Korea has to add to its nuclear arsenal.