However, the inspectors will have only partial access to the site, a senior Iranian official said on Thursday.
The team of inspectors from the International Atomic Energy Agency (IAEA) arrived in Iran on Wednesday to conduct a programme of inspections that include the Parchin military facility southeast of Tehran.
The IAEA has been waiting months for Iranian permission to access the site.
Iran denies that it has been trying to make nuclear weapons at Parchin or any other site in the country. It says its nuclear programme is geared solely to produce electricity.
"We are watchful. We have allowed inspections of our military installations, but we will not allow any espionage or the theft of information," said Hossein Mousavian, the main spokesman of Iran's nuclear negotiating team.
"It is not necessary for the inspectors to enter the buildings. They are authorised to take samples outside [the buildings] with their equipment," he told the semi-official Mehr news agency.
The IAEA has been investigating Iran's nuclear ambitions for more than two years.
"Undoubtedly the small countries in the region would turn to the United States if Iran seeks nuclear weapons"
Hossein Mousavian, Iran nuclear negotiating team spokesman
It has discovered several instances of undeclared activities that could be used to make atomic bomb fuel, but it has not found any conclusive proof that
Iran is trying to build nuclear weapons capability.
Iran says it has no intention of adding nuclear weapons to its military capability.
"Undoubtedly the small countries in the region would turn to the United States if Iran seeks nuclear weapons," Mousavian said.
"Iran's political, economic and social development would be jeopardised if Iran seeks to build nuclear weapons. We have never considered weapons of mass destruction to be a proper deterrent," he said.