The reports quoted diplomats in Vienna, who demanded anonymity in exchange for divulging the confidential information on Friday.

However, the diplomats cautioned that confirmation still had to come through other laboratory tests.

Initially, they said the density of enrichment appeared to be close to or above the level used to make nuclear warheads.

But later, a well-placed diplomat accredited to the International Atomic Energy Agency (IAEA) said it was below that level, although higher than the low-enriched material used to generate power.

Still, they said, further analysis could show that the find matches others established to have come from abroad.

The IAEA determined earlier traces of highly enriched uranium were imported on equipment from Pakistan that Iran bought on the black market during nearly two decades of clandestine activity discovered just over three years ago.

Uranium enriched to between 3.5% and 5% is used to make fuel for reactors to generate electricity. It becomes suitable for use in nuclear weapons when enriched to more than 90%.

Fissile material?

Because Iran has previously denied conducting enrichment-related activities at the site, the mere fact that the traces came from there could bolster arguments that it has hidden parts of a programme that can create the fissile material used in nuclear warheads.

Additionally, the site's connection to the military challenges Iranian arguments that their nuclear programme is purely civilian.

Ahmadinejad: The people of Iran
are not afraid of America

One of the diplomats told AP that the samples came from vacuum pumps that have various applications, including use in uranium-enriching centrifuges at a former research centre at Lavizan-Shian.

The centre is believed to have been the repository of equipment bought by the Iranian military that could be used in a nuclear weapons programme.

Despite their declared support for the European effort to persuade Iran to give up enrichment, the Americans are ignoring growing calls for direct contacts with Iran - a stance criticised on Friday by Kofi Annan, the UN secretary-general.

Calling on "all sides to lower the rhetoric", he said Washington should "come to the table" and join the Europeans and Iranians.

In a statement later on Friday, John Bolton, the US ambassador to the UN, said: "This is not a bilateral issue between the US and Iran. ... It is an issue between Iran and the world. ... We see no point in direct negotiations."

Military force

While Washington has said it favours a diplomatic end to the dispute, it has not ruled out military force and has been pushing for a tough Security Council resolution against Iran which could include sanctions.

However, several European countries are working on a plan which will present Iran with a choice of incentives or sanctions.

Meanwhile, Mahmoud Ahmadinejad, Iran's president, remains defiant. On Friday he accused the Americans of "waging a propaganda campaign" against his country.

"The people of Iran and the country are not afraid of them," he told Indonesian Islamic leaders in Jakarta, Indonesia, where he is on a state visit.