This file satellite image shows the military complex at Parchin, Iran, August 2004 (Photo: AP)
The UN atomic agency's chief inspector said he hopes Iran will grant his team access to the Parchin military base during talks in Tehran on Thursday.
"We also hope that Iran will allow us to go the site of Parchin, and if Iran would grant us access we would welcome that chance and we are ready to go," Herman Nackaerts told reporters at Vienna airport on Wednesday on his way to Tehran for the meeting.
The International Atomic Energy Agency (IAEA) says it has evidence suggesting Iran conducted explosives research at Parchin that would be applicable in making nuclear weapons.
Iran denies seeking or ever having sought nuclear weapons and has denied the IAEA access to Parchin, saying that as a non-nuclear site the agency has no right to conduct inspections there.
Tehran also says that the IAEA has already visited the site near Tehran twice, both times in 2005. The agency counters that since then, it has received additional information about work there that makes it want to go back.
The IAEA says "extensive activities" have been spotted by satellite at Parchin, such as the scraping and removal of earth over a 25-hectare (62-acre) area, leading to Western accusations that Iran is destroying evidence.
The alleged nuclear work at Parchin is part of a range of suspected activities summarised in a major IAEA report released in November 2011.
The report said the alleged evidence was "overall, credible" and that it indicated that until 2003, and possibly since, Iran carried out work "relevant to the development of a nuclear explosive device".
Because the bulk of the information in the report comes from foreign intelligence agencies, Iran has said it is either forged or related to non-nuclear work.
Thursday's meeting between Nackaerts' team and Iranian officials is the latest in a string of fruitless talks this year between the Iranians and the IAEA, which has pressed Iran to address the claims and grant access to Parchin and other sites and individuals.
The United States has warned that it will push for the board of the Vienna-based agency to refer Iran to the UN Security Council if Tehran displays no "substantive cooperation" by an IAEA board meeting in March.
Watching the talks closely will be six major powers keen to restart diplomatic efforts to resolve the decade-long and escalating crisis over Iran's nuclear programme.
Those efforts are focused more on Iran's current activities, in particular its expanding ability uranium enrichment to fissile purities of 20 percent.
This is close to the level needed for a weapon but which Tehran says is for nuclear medicines. Multiple UN Security Council resolutions have called on Iran to suspend all enrichment.