North Korea said yesterday it was closer to a second way of making nuclear weapons, a move analysts saw as a new tactic to put pressure on the international community after a month of conciliatory gestures.

The chief US envoy for the North, Stephen Bosworth, said the enrichment claim was "of concern", while Russia described the announcement as "very alarming".

Mr Bosworth was in Beijing on a trip to Asia to discuss ways to bring Pyongyang back to long-stalled negotiations on giving up its nuclear ambitions.

"Experimental uranium enrichment has successfully been conducted to enter into completion phase," the North's KCNA news agency quoted its United Nations delegation as saying in a letter to the head of the UN Security Council.

The US has long suspected the North of having a secret programme to enrich uranium for weapons. Experts have said it has not developed anything near a full-scale uranium programme while it has enough plutonium for six to eight bombs.

"I think for all of us, it reconfirms the necessity to maintain a coordinated position on the need for complete, verifiable denuclearisation of the Korean peninsula," Mr Bosworth said, after meetings with Chinese officials.

The North said its latest steps were in response to tighter sanctions.

"These reports cannot but provoke concern," Russia's Interfax news agency quoted an unidentified Foreign Ministry official as saying.

"A very alarming precedent is being created by such an open and demonstrative disdain for resolutions of the United Nations Security Council."

At the UN, the mission of the US, which currently holds the Security Council presidency, said it had received the North Korean letter and circulated it to the other 14 members of the council.