UN Secretary-General Ban Ki-moon said a crisis over North Korea had gone too far and urged dialogue and negotiation to resolve the situation.
On an official visit to Andorra yesterday, Mr Ban said: "Nuclear threats are not a game. Aggressive rhetoric and military posturing only result in counter-actions, and fuel fear and instability."
Earlier, North Korea's leader appeared to dampen down hostile rhetoric that had threatened impending war with the United States and South Korea.
The US has bolstered its forces in the region after a series of threats by Pyongyang to attack US bases in the Pacific and to invade South Korea.
Mr Ban offered to help the parties to begin talks, adding: "Things must calm down, as this situation, made worse by the lack of communication, could lead down a path that nobody should want to follow.
"I am convinced that nobody intends to attack [North Korea] because of disagreements about its political system or foreign policy. However, I am afraid that others will respond firmly to any direct military provocation."
He also called on North Korean authorities to abide by resolutions from the United Nations Security Council.
His comments came as the North announced plans to restart a mothballed nuclear reactor that has been closed since 2007, but emphasised it was seeking a deterrent capacity, rather than repeating recent threats to attack South Korea and the US. The state-owned KCNA news agency said North Korea would restart all nuclear facilities for both electricity and military uses.
North Korea conducted its third nuclear test in February but is believed to be some years away from developing nuclear weapons, although it claims to have a deterrent.
A speech by the North's young leader Kim Jong-un, delivered on Sunday but published in full by KCNA yesterday, appeared to play down the prospects of a direct confrontation with the US as he stressed nuclear weapons would ensure the country's safety as a deterrent.
Mr Kim said: "Our nuclear strength is a reliable war deterrent and a guarantee to protect our sovereignty. It is on the basis of a strong nuclear strength that peace and prosperity can exist and so can the happiness of people's lives."
Mr Kim's speech, delivered to the central committee meeting of the ruling Workers Party of Korea, appeared to signal a small shift from threats against South Korea and the US, but it was some distance from any kind of end to the crisis.
Yang Moo-jin of the University of North Korean Studies in Seoul said: "The fact this speech was made at the party central committee meeting, which is the highest policy-setting organ, indicates an attempt to highlight economic problems and shift the focus from security to the economy."
But if Pyongyang follows through with its plan to restart the nuclear facilities, it will have longer-term security implications for the region.
Reactivating the aged Soviet-era reactor at the Yongbyon nuclear plant will produce plutonium, a tested path to acquire more fissile material than a uranium enrichment programme.
It was unclear how quickly the Yongbyon plant, whose cooling tower was destroyed as part of a de-nuclearisation deal, would take to restart and it was impossible to verify whether it was still connected to North Korea's antiquated electricity grid at all.
The move to restart the reactor comes as a big blow to China's stated aim of restarting de-nuclearisation talks on the Korean peninsula, prompting a foreign ministry spokesman in Beijing to express regret at the decision.
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