The head of the UN's nuclear watchdog has urged Iran to allow international inspectors back into a key site.
The US says Iran might have test explosives at Parchin (photo: DigitalGlobe/Isis)
Mohamed ElBaradei said experts wanted to return to Parchin, which they first visited in January, and wanted more information about a site at Lavizan.
The US says Parchin could have been used to test explosive components for an atomic bomb.
Iran has strongly denied Washington's claims that it is pursuing a covert nuclear weapons programme.
A UN team did not get full access when they went to inspect Parchin, a centre for munitions work about 30km (19 miles) south-east of Tehran, and Iran has turned down requests for a repeat visit.
Speaking to the International Atomic Energy Agency's (IAEA) board of governors, Mr ElBaradei said information Iran had supplied about its nuclear activities was "not yet sufficient to answer several remaining questions.
"I would... ask Iran to support the agency's efforts to pursue further its investigation of the Lavizan-Shian and Parchin sites," he said.
Mr ElBaradei, who was appointed for a third four-year term on Monday, said Iran was continuing its suspension of nuclear enrichment activities, in line with the deal Tehran made with three European countries last year.
Iran is seeking closure of the two-year investigation into its nuclear programme, which it says is solely for peaceful civilian purposes.
Mr ElBaradei said the investigation was "inching forward" and that "the ball is very much in Iran's court".