The International Atomic Energy Agency says significant progress has been made in its probe into the possible military dimensions of Iran's nuclear programme.
IAEA director-general Yukiya Amano said Iran had handed over environmental samples from the Parchin military site, which he was able to visit on Sunday.
But Mr Amano cautioned that "much work" remained before the investigation could be completed by December's deadline.
Western powers suspect that nuclear weapons research took place at Parchin.
But Iran has denied this and insists that its nuclear programme is entirely peaceful.
The completion of the IAEA's investigation is crucial to the implementation of July's comprehensive agreement between six world powers and Iran, under which the country will limit its sensitive nuclear activities in return for the lifting of sanctions.
'Places of interest'
The complex at Parchin, south of Tehran, is dedicated to the research, development and production of ammunition, rockets and explosives.
Concerns about its possible role in Iran's nuclear programme emerged in 2004, when reports surfaced that a large explosives containment vessel had been built there to conduct hydrodynamic experiments. The IAEA says they are "strong indicators" of possible nuclear weapon development.
Inspectors did not observe any unusual activities during a visit in 2005 and environmental samples did not indicate the presence of nuclear material, but suspicions persisted.
Until Sunday the Iranian government had repeatedly rejected requests by the IAEA to revisit Parchin, and extensive landscaping, demolition and new construction was observed.
Mr Amano was given a tour of Parchin on Sunday along with the head of the IAEA's Department of Safeguards, Tero Varjoranta.
At a news conference in Vienna on Monday, Mr Amano said they were able to inspect a building that the agency had previously only observed using satellite imagery. They found no equipment inside, but did see indications of recent renovation work.
Mr Amano also said environmental samples were taken from "places of interest" at Parchin last week, and that "the Iranian side played a part in the sample-taking process by swiping samples".
He did not reveal whether IAEA inspectors were present at the time of the procedure, but stressed it was carried out in a way that verification processes were "not compromised".
"The agency can confirm the integrity of the sampling process and the authenticity of the samples," he said. "The process was carried out under our responsibility and monitoring."
The collection of samples at Parchin by Iranian experts is believed to be part of a confidential "road map" agreement signed by the IAEA and Iran in July.