US 'concern' over N Korea uranium
The US says it is "very concerned" at reports that North Korea has entered the final phase of uranium enrichment, BBC reported.
The White House said it would "strongly implement" tougher sanctions passed by the United Nations after a nuclear test conducted by Pyongyang in May.
The US said it wanted a denuclearised Korean peninsula.
Uranium enrichment would give Pyongyang a second way to make a nuclear bomb. The North says it is also continuing to weaponise plutonium.
US state department spokesman Ian Kelly said it was unclear how true the latest statements from North Korea were.
But he added: "In general, we are very concerned by these claims that they're moving closer to the weaponisation of nuclear materials."
Earlier, White House spokesman Robert Gibbs said: "We continue to be committed to ensuring that North Korea upholds its international obligations and we continue to strongly implement the sanctions that were approved."
British Foreign Secretary David Miliband said there needed to be a unified international response to North Korea's moves.
He said: "It shows that 2009 and 2010 are the years when the Non-Proliferation Treaty is being tested as never before and when there needs to be extra drive from all of us."
The two tests carried out by the North - in May and in 2006 - were understood to have been carried out with plutonium.
The BBC's John Sudworth in South Korea says the worry is that uranium enrichment is a process that can be easily hidden, and in addition, North Korea has ample natural reserves of the raw material.
The North's KCNA state media said Pyongyang's delegation at the UN had written to the Security Council on the issue.
"If some permanent members of the UN Security Council wish to put sanctions first before dialogue, we would respond with bolstering our nuclear deterrence first before we meet them in a dialogue," the delegation wrote.
South Korea's foreign ministry condemned what it calls "threats and provocative acts".
North Korea's plutonium programme is based on the Yongbyon reactor, which is under US satellite observation.
Observers say the US has long suspected the existence of a secret uranium enrichment programme in the North, though experts say it remains little-developed.