United Nations nuclear inspectors have been allowed to visit Iran's sensitive Parchin military complex.
The US says Parchin is suspicious (photo: DigitalGlobe/Isis)
The International Atomic Energy Agency (IAEA) is trying to check US claims that tests linked to nuclear weapons' development have taken place there.
The agency has threatened to refer Tehran to the UN Security Council for possible sanctions unless its inspectors get full co-operation.
Iran is under pressure over its nuclear programme, which it says is peaceful.
Parchin is a centre for munitions work about 30km (19 miles) south-east of Tehran, which the IAEA has sought to re-inspect since it was last there in January this year.
"We are pleased that we can confirm that IAEA inspectors got access to buildings at the Parchin site as we had requested," a spokesman for the Vienna-based agency said.
A diplomat close to the IAEA told reporters that environmental samples had been taken from the complex.
Iran has also handed over documents and granted interviews with several senior officials thought linked to black market purchases of uranium enrichment technology, according to one diplomat quoted by the Associated Press.
A Western diplomat told the BBC the move to grant access to Parchin was a positive sign that Iran was responding to intense international pressure.
But Iran has also told the IAEA that it plans to continue processing uranium within weeks.
Tehran insists its uranium processing programme is designed to meet its energy needs.
But enriched uranium could be used to produce either nuclear energy or nuclear weapons.
If Tehran continues with the process, say diplomats, so will the suspicion that it is trying to obtain nuclear weapons.