North Korea has said it has no choice but to increase its nuclear deterrent following multilateral talks in Beijing.
Initial good will seems to have quickly evaporated
The statement, made by a North Korean Foreign Ministry spokesman, also said Pyongyang is not interested in holding any more talks on the future of its controversial nuclear programme.
"The talks have made us believe that we have no other choice but to strengthen a nuclear deterrent force as a self-defensive means," the unnamed spokesman told the North's official KCNA news agency.
Washington had said it was "pleased" at the outcome of the three-day talks in the Chinese capital , which involved the US, North and South Korea, Russia, China and Japan.
As the talks ended on Friday, all the parties reportedly agreed to meet again within two months.
Although there was no joint statement, the participants had agreed that the Korean peninsula should be nuclear free.
It was also thought they had reached a consensus not to escalate the situation while a solution is being negotiated.
Analysts say that Pyongyang's surprise statement on Saturday should be not taken too literally as it may be part of its usual negotiating tactics.
Some even dismissed the threat as mere posturing.
Tensions have been high since the US and North Korea became engaged in a war of words over Pyongyang's nuclear ambitions last October.
'Long way to go'
North Korea has called for economic aid and a non-aggression pact with America in return for surrendering its nuclear ambitions, but Washington has consistently refused.
Shortly before the North Korean statement, US Assistant Secretary of State James Kelly had appeared reasonably satisfied with the Beijing talks.
"We had a nice visit and a productive start. We have a long way to travel and don't know when we will be back here or whether it will be somewhere else.
"But a peaceful solution is something we are going to work on," he said.
The BBC's correspondent at the talks, Charles Scanlon, had said an agreement to meet again had been seen as the best that could be achieved.
The crisis flared last October when the US said North Korea had admitted to a covert nuclear weapons programme which violated a 1994 agreement between the two countries.
Washington has characterised North Korea as part of an "axis of evil" and has said North Korea has admitted to possessing at least one nuclear bomb.
The US and its allies suspended fuel aid to North Korea, and North Korea responded by reactivating a mothballed nuclear plant, expelling United Nations weapons inspectors, and pulling out of the international Nuclear Non-Proliferation Treaty (NPT).
In a BBC interview, the head of the UN nuclear watchdog, Mohamed ElBaradei, said North Korea had been guilty of nuclear "blackmail".
"I don't think they can be trusted," said the head of the International Atomic Energy Agency (IAEA). "However, we would like to work with them and bring them back to the Non-Proliferation Treaty."