Inspectors from the International Atomic Energy Agency have postponed a trip to Iran after the government requested more preparation time.
Iran denies it has a secret weapons programme
The team, who had been due to arrive in Tehran on Sunday, will now leave sometime late next week.
The United States has been pressuring the IAEA over Iran, which it alleges is trying to develop nuclear weapons.
Iran denies the charge and has been given until 31 October by the IAEA to prove it has no such secret programme.
In announcing the postponement IAEA spokeswoman Melissa Fleming gave no details of why the request for more time had been made.
The decision comes a day after a report from the United Nation's nuclear agency said that a team of experts had discovered more traces of highly enriched uranium in Iran on a prior visit.
IAEA chief Muhammad ElBaradei has urged Iran to co-operate
Diplomats said minute quantities of uranium were found last month at the Kalaye Electric Company on the southern outskirts of the capital, Tehran.
One of the diplomats at the IAEA's headquarters in Vienna told the BBC that it was unclear whether the weapons-grade material was produced by Iran or the result of contamination from imported equipment.
Earlier this year IAEA officials said they found enriched uranium particles at a plant at Natanz, which the Bush administration says is a clear indication that the Islamic state is secretly trying to develop nuclear weapons.
Tehran says its nuclear programmes are peaceful and that it only wants to produce low-enriched uranium, unusable in bombs.
It says the Kalaye Electric Company site is not part of its nuclear power programme but has been used for storing equipment.
The IAEA's board of governors has set a 31 October deadline for Iran to disprove the US claims.
On Thursday US President George W Bush, who previously branded Iran part of an "axis of evil" alongside Iraq and North Korea, warned Tehran that it faced "universal condemnation" if it was indeed developing such weapons.
"People understand the danger of Iranians having a nuclear weapon programme," he said, referring to his talks with world leaders attending the UN General Assembly earlier in the week in New York.