The US will not accept a North Korea armed with nuclear weapons, a top US envoy has said, after Pyongyang announced plans for a nuclear test.
Pyongyang's nuclear plans have aroused anger in South Korea
North Korea must choose either to have a future or to have nuclear weapons "but it cannot have them both", top US negotiator Christopher Hill said.
He did not specify how the US would respond if a nuclear test took place.
The US wants other countries to unite against the test plans, but talks at the UN have been inconclusive.
"At this stage, there's division," said John Bolton, US envoy to the UN.
Japan was leading efforts to agree a strongly-worded UN statement.
But China has appealed for calm, saying it hopes North Korea will "exercise the necessary calm and restraint", and calling for the issue to be handled in revived six-nation talks.
Russia and South Korea have said that North Korea's plans to conduct a nuclear test are unacceptable.
Their foreign ministers, Sergei Lavrov and Ban Ki-moon, agreed in a telephone conversation that a test would only aggravate the situation, Russia said.
The new Japanese Prime Minister, Shinzo Abe, is to visit China and South Korea in the next few days to discuss the crisis.
Mr Hill, Washington's top envoy at the stalled six-party talks with North Korea, said the US was rallying its allies in a diplomatic push against Pyongyang.
KOREAN NUCLEAR CRISIS
Sept 2005:At first hailed as a breakthrough, North Korea agrees to give up nuclear activities
Next day, N Korea says it will not scrap its activities unless it gets a civilian nuclear reactor
US imposes financial sanctions on N Korea businesses
July 2006: N Korea test-fires seven missiles
UN Security Council votes to impose sanctions over the tests
Oct 2006: N Korea threatens nuclear test
"I am not prepared at this point to say what we are going to do but I am prepared to say we are not going to wait for a nuclear North Korea, we are not going to accept it," he said.
He said North Korea had reached "a very important fork in the road - it can have a future or it can have these [nuclear] weapons but it cannot have them both".
Mr Hill said the message had been conveyed to Pyongyang's envoy at the UN but had yet to elicit a response.
The test threat has also caused alarm in South Korea. South Korean Vice-Foreign Minister Yu Myung-hwan warned that a successful North Korean test could trigger an Asian arms race.
A test "could provide a pretext for Japan's nuclear armament," he said.
"This will prompt countermoves by China or Russia and lead to a change in the balance of power in north-east Asia," he said.
North Korea announced its plans for a nuclear bomb test on Tuesday, saying it would boost security in the face of US hostility, though it did not set a date for a test.
It is thought to have developed a handful of warheads but never before announced it would test one.
US and South Korean reports suggest the North has at least one underground test site.
The North appears increasingly angry at sanctions imposed by the US and other countries on North Korean businesses accused of arms sales and illegal activities.
In 2002, it restarted its nuclear reactor at Yongbyon and forced two UN nuclear monitors to leave the country. It is unclear how far work has progressed at the plant since then.