VIENNA - The U.S. dismissed Wednesday a recent offer by Iran to discuss its disputed nuclear program with world powers as a "charm offensive," saying Iran was "flouting" its international obligations.

The statement by Glyn Davies, chief U.S. delegate to the International Atomic Energy Agency, comes ahead of next week's UN General Assembly meeting. Davies said the Islamic republic's latest offer for talks fails to provide any fresh indications of an Iranian commitment to address international concerns that it could be working on nuclear warhead experiments.

"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," Davies told reporters.

Iran already is under four sets of Security Council sanctions for refusing to freeze uranium enrichment, as the U.S. and its Western allies have been demanding for years.

Tehran says it needs the program to make reactor fuel, but the council fears it could be re-engineered to produce fissile warhead material -- despite Iranian insistence that its program is purely for peaceful purposes.

In a letter dated Sept. 6 and addressed to EU foreign policy chief Catherine Ashton, Iran suggested a fresh round of talks but gave few details of what it would be ready to discuss with the six countries involved in the negotiations, the United States, Russia, China, Britain, France and Germany.

In its latest Iran report earlier this month, the IAEA for the first time said it is "increasingly concerned" about credible "extensive and comprehensive" intelligence suggesting that Iran continues its secret weapons work. At the opening of a meeting of the IAEA's 35 board members earlier this week, the agency's chief Yukiya Amano indicated the body had information backing this belief.

Iran's ambassador to the IAEA fired back by insisting that no one has any evidence of nuclear materials being used for military purposes in his nation.

"There is no smoking gun," Ali Asghar Soltanieh told reporters. "The EU and the U.S. cannot dictate to us what to do."

He further pointed to a visit in August by the agency's safeguards chief to several of Iran's nuclear facilities as evidence of his nation's "100 per cent transparency and political will for co-operation with the agency" regarding questions over its nuclear program.

But a trio of EU nations involved in the ongoing negotiations with Iran said the agency's report indicated Iran is "advancing in an extremely concerning direction" and echoed the U.S. view that Iran was falling short of necessary compliance.

"In our view, a visit is not enough," Britain, France and Germany said in a joint statement. "This has been a selective and insufficient transparency."