Diplomats say international inspectors have found new traces of highly enriched uranium in Iran that could indicate Tehran is secretly seeking material for nuclear weapons.
News agencies quote the diplomats, speaking anonymously, as saying the U.N. atomic energy agency took samples from equipment used at a former research center Lavizan in Iran.
The agency wrote, in a report to the U.N. in April, that the swabs taken from vacuum pumps used at the site turned up the traces.
Highly enriched uranium is required for nuclear weapons.
The sources said there could be other explanations, including contamination of equipment bought from abroad, but Iranian officials had not reported nuclear enrichment activity at the site.
In other news, U.N. Secretary-General Kofi Annan is urging the United States to join European countries in direct talks with Iran to end the standoff over Tehran's nuclear program.
Iran says it is enriching uranium for peaceful purposes.
The U.S. has refused to take part in direct talks, but has given its European allies some time to persuade Iran to halt all uranium enrichment activity or face possible U.N. sanctions.
Three EU countries - Britain, France and Germany - are preparing a package of incentives for Iran to give up its nuclear ambitions.
The package is expected to be presented to Tehran within weeks. Iranian President Mahmoud Ahmadinejad said today the nuclear standoff is "just psychological propaganda" aimed at threatening his country.
Information for this report is provided by REUTERS and AFP.