The United States warned last night it would not live with a nuclear-armed North Korea.
"We are not going to live with a nuclear North Korea, we are not going to accept it," US Assistant Secretary of State Chris Hill said.
US Assistant Secretary of State Chris Hill
He warned Pyongyang that "it can have a future or it can have these weapons. It cannot have both".
Mr Hill spoke after a US defence official said the United States had detected activity at potential test sites in North Korea, indicating possible preparations for a nuclear test.
On Tuesday, North Korea, which has said it has nuclear bombs, said it would conduct its first nuclear test. Mr Hill refused to say what steps Washington might take to see that North Korea did not succeed in testing a weapon.
But he said: "We will do all we can to dissuade the DPRK from this test.
"We would have no choice but to act resolutely to make sure that the DPRK understood - and to make sure that any other country understands - that this [nuclear test] is a very bad mistake," Mr Hill said.
At the United Nations, Japan pushed for a Security Council warning that if North Korea went ahead with a test, the council would impose consequences, although it did not specify what those should be.
US Ambassador John Bolton said that while Japan, Britain and France made clear a strong statement was needed, he was not certain "what North Korea's protectors" on the 15-member council would do.
In response, China's UN Ambassador Wang Guangya said: "I'm not sure which country he is referring to, but I think that for bad behaviour in this world no one is going to protect them [North Korea]."
The Foreign Ministry of China, the closest North Korea has to an ally, called for restraint on North Korea's part and for other countries to avoid actions that would heighten tensions.