Herman Nackaerts, the leader of the five-member U.N. team, said he wanted concrete results from the two-day visit, the second within a month. But, amid skepticism that inspectors would be permitted access to nuclear facilities, Nackaerts added that progress "may take a while".

The team is hoping to question Iranian nuclear scientists and visit the Parchin military base, where high-explosive tests are thought to have been conducted. But Ali Akbar Salehi, Iran's foreign minister, told a student news agency that the officials would not be inspecting any sites.

In a sign of mounting tensions in the region, Iran began a four-day military exercise in the south of the country to "practice co-ordination between the Revolutionary Guards and regular army and air defense units in establishing a defense umbrella over our vital centers, particularly nuclear facilities", according to a military statement quoted by an Iranian news agency.

Missiles, anti-aircraft artillery, radars and warplanes were being deployed, it said.