UN Secretary-General Ban Ki-moon said in response to Iran's announcement that it had begun enriching uranium using 3,000 centrifuges
that he hoped Iran would "engage in dialogue...It is very important for any member country to fully comply with the Security Council resolution."
The British government expressed concern Monday over Iran's announcement that it has dramatically expanded its nuclear program.
In London, the British Foreign Office said Iran should suspend uranium enrichment immediately.
"We are concerned that Iran appears to have confirmed more nuclear enrichment activity" in violation of UN resolutions, the ministry said in a statement.
"We have always made it clear that we are happy for Iran to have a civil nuclear power program, but they have to satisfy the international community that that is what it is seeking to develop.
"The international community stands together in ensuring that Iran does not develop the means to acquire nuclear weapons."
Earlier, the US State Department said that Iran's decision to start industrial scale enrichment of uranium was a further act of defiance of the international community's demands that Teheran end its nuclear program.
Spokesman Sean McCormack said Iran's actions are such that the UN Security Council and the UN nuclear watchdog group "don't believe Iran's assurances that their (nuclear) program is peaceful in nature."
He noted that there will be a UN International Atomic Energy Agency report on May 24 about what agency officials have found in monitoring Iranian nuclear activities.
He also recalled that the Security Council has approved two punitive resolutions against Iran, and he expressed regret that these actions have not yet had the effect of changing Iran's behavior.
"What we are looking for are reasonable Iranian leaders who view the cost-benefit calculation and see that it is not to the benefit of the Iranian people to continue to pursue the course on which they find themselves," he said.
At the White House, National Security Council spokesman Gordon Johndroe called the Iranian decision unacceptable.
The actions deny the Iranian people the benefits they otherwise might enjoy under the incentives package offered by the United States, Britain, France, Russia, China and Germany, Johndroe said.
"We call on the Iranian regime to comply with its obligations to the IAEA and UN Security Council," he said.
The German Foreign Ministry said that the move showed Iran was "definitively going in the wrong direction."