The statement came in the wake of the United States, in its capacity as the President of the Security Council for the month of September, receiving a letter from North Korea informing it that it is in final stages of the uranium enrichment process.

"The letter we received in our capacity as the President of the UN Security Council. I think that in terms of characterising it, I would say that it just reconfirms the need for North Korea to step back from any kind of nuclear weapons programme and recommit to the obligations that it made in the September 2005 joint statement. And that was the complete and verifiable denuclearisation of the Korean Peninsula," State Department spokesman Ian Kelly said.

"We would just urge them to return to the table and join us with our Six-Party partners to resume talks leading to our goal," Kelly said.

On a question, he said: "We are very concerned by these claims that they are moving closer to the weaponisation of nuclear materials."

Kelly said the US is very concerned about the claims made by North Korea, but is not in a position to confirm that these claims were true.