VIENNA - Western powers said on Wednesday there was growing evidence suggesting Iran was working to develop a nuclear missile and a recent "charm offensive" by Tehran failed to address those fears.

Statements by Britain, Germany, France and the United States at a board meeting of the UN nuclear watchdog made clear they were not impressed with an Iranian effort to show increased openness about its disputed atomic activities.

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"Iran continues to casually dismiss the international community's concerns," Glyn Davies, the US envoy to the International Atomic Energy Agency, said.

"Stonewalling the IAEA, flouting UN Security Council obligations and mounting this most recent charm offensive do not reflect a good faith effort to resolve those concerns."

In a hard-hitting joint statement, European Union heavyweights Britain, France and Germany said Iran's nuclear programme was "advancing in an extremely concerning direction".

Like the United States, they voiced particular alarm at Iran's decision to move higher-grade uranium enrichment to an underground bunker - heightening their suspicions of its aims.

"The absence of a plausible economic or commercial rationale for so many of the nuclear activities now being carried out in Iran, and the growing body of evidence of a military dimension to these activities give grounds for grave concern about Iran's intentions," British Ambassador Simon Smith said on behalf of London, Berlin and Paris.

Davies said current IAEA monitoring of Iranian nuclear sites might provide some warning should Iran decide to "break out" and use its enriched uranium stockpile to develop nuclear bomb capability, but "that will come too late."

Together with China and Russia, the four Western states make up the six powers which have long sought - so far in vain - to find a diplomatic solution to a dispute that has the potential to spark a wider conflict in the Middle East.

Iran denies Western accusations its program is aimed at developing a nuclear weapons capability.

But its refusal to suspend enrichment, clarify foreign intelligence reports pointing to possible atom bomb research and grant unfettered access for IAEA inspectors has drawn tightening UN and Western sanctions against the major oil producer.

Uranium enriched to a low level of fissile purity is suitable for running civilian nuclear power plants. If refined to a much higher degree, it can form the core of nuclear bombs.

The IAEA has added independent pressure on Iran, with its director, Yukiya Amano, saying publicly for the first time this week that he was "increasingly concerned" about possible military aspects of Iran's nuclear programme.

Amano also said he planned to present soon the basis for those concerns in more detail to member states, a step that could provide stronger arguments for Western punitive measures.

Vienna-based diplomats say this may be why Iran -- which says its nuclear work is aimed at producing electricity -- is now showing more willingness to engage with the U.N. body.

In August, Iran allowed a senior IAEA inspector access to two nuclear-related sites in the country that the UN agency had not had access to for several years, saying this showed Tehran's "100 percent transparency and openness."

Last week, its chief nuclear negotiator Saeed Jalili sent a letter to European Union Foreign policy chief Catherine Ashton expressing readiness to resume stalled talks with the powers.

But Jalili also made clear Iran would not back down over its nuclear "rights" - language that usually refers to uranium enrichment, among other atomic fuel cycle activities.

Davies said the six powers were discussing how to respond to Iran's letter to Ashton, who is handling diplomatic contacts with Tehran on behalf of the six capitals, and he suggested they may decide to engage with the Islamic Republic as a result.

But he said the Iranian letter did not contain any new commitments by Iran to address international suspicions.

"I don't see from the standpoint of the work that is going on here anything new by way of an Iranian commitment to fully address the concerns that the international community has," Davies told reporters on the sidelines of the IAEA meeting.

Iran has often said it is willing to resume talks. But its insistence that other countries recognise its right to enrich uranium is a major stumbling block, particularly for Western diplomats who see it as an unacceptable precondition.

Since negotiations between the powers and Iran foundered in January, Russia has advocated a phased plan in which Tehran would address concerns that it may be seeking nuclear weapons, and be rewarded with an easing of sanctions.

"We note Iran's recent claim that it is starting a new era of cooperation," Davies told the 35-nation IAEA board. "We have heard this claim before, but it has yet to be fulfilled."

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