Agence France Presse - Britain said Wednesday that Iran's claim of success in enriching uranium was "deeply unhelpful" and warned that the United Nations could respond with "diplomatic measures".

"I am seriously concerned," said Foreign Secretary Jack Straw, reacting to Iranian President Mahmoud Ahmadinejad's announcement the day before which upped the stakes in Tehran's nuclear showdown with the West.

"It is contrary to repeated requests by the International Atomic Energy Agency (IAEA) board, and now by the Security Council, that Iran resume full and sustained suspension of all enrichment and reprocessing activities, including research and development," he said in a statement.

"The Iranian regime must demonstrate by its words and actions that it is not seeking nuclear weapons," he said, adding: "The latest Iranian statement further undermines international confidence in the Iranian regime and is deeply unhelpful."

Straw recalled that IAEA director general Mohamed ElBaradei is meeting Iranian officials in Tehran this week, before reporting back to the UN Security Council at the end of this month.

Oil-rich Iran insists its nuclear programme is intended for civilian energy purposes only. But its failure to cooperate fully with IAEA inspectors has prevented the UN agency from ruling out secret military intentions.

Straw said: "If Iran does not comply (with UN demands), the Security Council will discuss further diplomatic measures."

"I call upon Iran to suspend its activities, begin the process of building confidence, and get back into negotiations," he added.

"We will remain in close contact with our international partners, whose strong reactions show the depth of international concern there is about Iran's activities."