A diplomat close to the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said that UN inspectors had visited sites related to the Lavizan military complex and seen equipment.
Iran has removed buildings and topsoil from Lavizan but IAEA inspectors had long wanted to investigate Lavizan machinery which could be used for either civilian or weapons purposes.
Along with criticism of Iran's resumption of enrichment-related activities, the UN nuclear watchdog has criticised Tehran for lack of access to monitor its program.
The recent access comes ahead of a meeting in London of foreign ministers from the United States, Britain, France, Germany, Russia and China who will attempt to attempt to find agreement on how best to respond to Iran's resumption of sensitive nuclear activities.
Diplomats and analysts said the last minute Iranian access is consistent with its past behaviour of making concessions when faced with an international crackdown.
But non-proliferation analyst Gary Samore from the MacArthur Foundation in Chicago has warned that Iran "uses cooperation as a tool".
"Rather than fully cooperate, they try to dole out small portions of cooperation and to hold out in other areas where they may be hiding things," he said.
The Europeans and the United States are pushing for a stern resolution at the IAEA meeting that declares the UN body no longer has confidence in Iran's intentions regarding its nuclear program.
Russia and China, which have strong trade ties with Iran, want to have a decision postponed for one month to allow for further diplomacy.
The European trio and the United States want to take Iran before the UN Security Council, which has enforcement powers such as sanctions, to pressure Tehran to cease all nuclear fuel work and to comply fully with a now three-year-old IAEA investigation into Tehran's atomic program, which the United States claims hides secret weapons work.