N Korea warned over nuclear test
A North Korean nuclear test would be a "provocative act", US Secretary of State Condoleezza Rice has said.
Speaking in Cairo, she said the US would have to assess its options should North Korea's threat be carried out.
North Korea did not give a date for its test, which was announced on state TV, but said it would boost security in the face of US military hostility.
Japan, South Korea and Russia, who have been involved in six-party talks with Pyongyang, have also expressed concern.
The US and China have also been part of the talks, which have been stalled for a year.
No reaction to the news was immediately available from China, one of North Korea's few allies.
The Chinese have, like the South Koreans, been advocating quiet diplomacy in efforts to get Pyongyang to abandon its nuclear programme, but both countries joined in condemning ballistic missile tests it conducted in the Sea of Japan earlier this year.
The latest announcement from Pyongyang is bound to put both Beijing and Seoul under renewed pressure to take a tougher line, BBC East Asia regional editor Clare Harkey says.
She adds that a nuclear test by North Korea, with its crumbling infrastructure and struggling economy, would also raise fears on safety grounds.
The US ambassador to the UN, John Bolton, said he would be raising the nuclear issue with the Security Council and hoped for a strong response similar to the one following missile tests.
"Given the very strong action by the council in July in condemning the North Korean ballistic missile tests, I think it's important that we're prepared to follow up here," he said.
Japanese Prime Minister Shinzo Abe said the nuclear test plans were unacceptable and would be met "harshly".
South Korea's government held emergency security talks, while Russian Foreign Minister Sergei Lavrov called for restraint and a diplomatic solution.
In London, Foreign Office officials warned of "serious consequences" to what would be regarded as "a highly provocative act".
North Korea 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.
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