"We told the Europeans that we consider this to be really an act of bad faith and to deliver a stern message to Iran that they must stop this," the official said.

The official said the United States was waiting to see "what kind of message the Europeans will send," with the message to be separate from a resolution the European trio was drafting for a key meeting in Vienna Thursday of the UN nuclear watchdog on Iran's nuclear program.

The US State Department said earlier in Washington that it was seriously concerned by new reports that Iran was producing the uranium feedstuff that could be used to make nuclear weapons just days before it is due to introduce a promised ban on all such enrichment activities.

The State Department was not able to substantiate accusations that Iran had converted 37 tonnes of uranium yellowcake into an unknown amount of uranium hexafluoride gas, but said that if the allegations were true they would further erode Tehran's already shakey credibility on the matter.

Uranium hexafluoride gas is used as the feed to make enriched uranium.

"Obviously, they are of concern," deputy spokesman Adam Ereli told reporters. "If true, they would yet again raise serious concerns about Iranian good faith and intentions.

"These allegations only heighten our concerns that Iran continues to pursue nuclear activities and does not honor its commitments," he said.

The United States charges that Iran is secretly developing nuclear weapons and wants the IAEA to take Tehran to the UN Security Council, which could impose punishing sanctions.

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