"Today, we went to Lavizan, we took environmental samples, we did all we wanted to. We got a prompt response for access from the Iranians," International Atomic Energy Agency (IAEA) director general ElBaradei told AFP.
ElBaradei, who is in Moscow for a conference on the peaceful use of nuclear energy, said the IAEA inspectors had seen "relevant equipment and they will be going to another site tomorrow to see more equipment."
He refused to say specifically what equipment was involved. But there have been reports that satellites have noticed a radiation body counter machine at the site.
"I think we got prompt access and I'm pleased with the prompt access. We did all we wanted to do," said ElBaradei, who at an IAEA meeting earlier this month had criticized the Iranians for giving "less than satisfactory" cooperation in helping the IAEA answer US charges that Iran is secretly developing nuclear weapons.
Diplomats confirmed to AFP a report that the IAEA had received information about concealment activity at an alleged nuclear site in Lavizan Shiyan, a Tehran neighbourhood.
The site was brought to public attention in May last year, when an Iranian opposition group, the National Council of the Resistance of Iran, alleged that the site was home to a biological weapons research facility.
The United States claims that Iran is secretly trying to develop nuclear weapons and has urged the IAEA to cite Tehran before the UN Security Council for possible sanctions.
The 35-nation board of the IAEA passed a resolution on June 18 rebuking Tehran for failing to come clean about its nuclear program, deploring its lack of cooperation and calling for the investigation into Iran's nuclear activities to be wrapped up within a few months.