United Nations nuclear inspectors have visited a military site in Iran which the US says may be linked to a secret nuclear weapons programme.
The US says satellite images of Parchin point to suspicious activity (photo: DigitalGlobe/Isis)
They went to the Parchin base near Tehran, to see whether nuclear material had been tested there.
According to US experts, satellite images of the site suggest that some buildings may be used to test nuclear bomb components.
Tehran denies the claim and insists its nuclear activities are peaceful.
Speaking ahead of the visit by International Atomic Energy Agency (IAEA) officials, Hossein Mousavian of Iran's nuclear negotiations team said the inspectors would only have partial access to the site.
"We are watchful. We have allowed inspections into our military installations but we will not allow any espionage or the theft of information from our military sites," he said, according to the Mehr news agency.
"It is not necessary for the inspectors to enter the installations. They are authorised to take samples outside (the buildings) using their equipment."
The IAEA has not reported any restrictions during the visit. Its spokesman, Mark Gwozdecky, would only say that the team had taken environmental samples.
Iran maintains that its nuclear programme is strictly civilian, and that it is not developing atomic weapons.
But the US suspects Tehran has a covert nuclear weapons programme, and wants the IAEA to take Iran to the UN Security Council for possible sanctions.
US officials increased the pressure after the publication of satellite images of the Parchin site by the Institute for Science and International Security last September.
A senior American delegate at the Vienna-based IAEA said it "clearly shows the intention to develop weapons".
The European Union resumed trade talks with Iran on Wednesday, 18 months after they were halted over Tehran's nuclear programme.
Negotiations restarted after Iran agreed in November to suspend the enrichment of uranium.