UN nuclear inspectors in Iran have reportedly discovered undeclared designs for a key machine in the production of bomb-grade material.
Tehran denies it has a nuclear weapons programme
The uranium centrifuge designs resemble a European model, Western diplomats told Reuters news agency.
A diplomat told the BBC that the news did not help Iran's credibility.
The US has again accused Iran of pursuing nuclear weapons, while the Islamic republic insists its nuclear programme is for civilian purposes.
"There's no doubt in our mind that Iran continues to pursue
a nuclear weapons programme," US Under-secretary of State John
Bolton told a security conference in Berlin.
"They have not yet, in our judgment, complied even with the commitments they made in October to suspend their uranium enrichment activities."
On Wednesday, US President George W Bush said international treaties intended to regulate the development of nuclear power must be strengthened to stop countries producing material which could be used for weapons.
North Korea and Iran had both done this by exploiting loopholes allowing the enrichment and reprocessing of uranium for peaceful purposes, he said - and this had to be stopped.
IAEA under pressure
The diplomats who spoke to Reuters in Vienna - home of the UN's International Atomic Energy Agency - said IAEA inspectors had uncovered centrifuge blueprints.
Last November Iran agreed to suspend its uranium enrichment programme and permit tougher IAEA inspections of its nuclear facilities.
An IAEA report the same month said Iran had admitted producing high-grade plutonium for peaceful purposes but the organisation concluded there was no evidence of a weapons programme.
In December, Tehran signed a UN agreement which allowed tougher inspections of its nuclear industry.
President Bush criticised the IAEA's methods, saying countries like Iran, who are suspected of breaking the rules, should not be allowed to sit on the committees that enforce them.
The IAEA's head, Mohamed ElBaradei, has said he agrees that quick action is needed to stop terrorists getting hold of nuclear weapons.
But, in an article for the New York Times newspaper, he said nuclear powers such as the US, along with Britain, France, Russia and China, should themselves "move towards disarmament".