By Bethany Bell
BBC News, Vienna
The United Nations nuclear watchdog, the IAEA, wants its inspectors to return to Iran's Parchin military site.
The US says Parchin shows suspicious structures (photo: DigitalGlobe/Isis)
The United States says it believes work on a secret nuclear weapons programme may have been carried out there.
IAEA inspectors went to Parchin last week, several months after first asking Iran for permission to visit it.
They took environmental samples to test for traces of nuclear materials but did not get full access to the site.
Now diplomats say the inspectors want to visit another part of the Parchin complex.
The US accuses Iran of trying to develop nuclear weapons.
It suspects that the Parchin site - a centre for munitions work - could have been used to test explosive components for an atomic bomb.
Iran denies the charge, saying its nuclear activities are entirely for peaceful purposes.
So far, the IAEA has found no clear evidence of a nuclear weapons programme in Iran but the head of the agency, Mohamed ElBaradei, says the jury is still out on the matter.