Iran has expanded its uranium enrichment activities in recent months, the International Atomic Energy Agency (IAEA) said in its latest report, obtained by news agencies on Friday, on the country's disputed nuclear program.

The restricted quarterly report circulated to IAEA member states showed Iran had increased activities at its main enrichment plant near the central city of Natanz, as well as work at the Fordow underground site near the city of Qum.

Meanwhile, the UN nuclear watchdog also said the "agency continues to have serious concerns regarding possible military dimensions to Iran's nuclear programme."

According to media reports, the IAEA said 52 cascades at Natanz, - each containing around 170 centrifuges - were now operating, up from 37 in November.

At Fordow, considered by some military experts as air raids-proof, almost 700 centrifuges are now refining uranium to a fissile concentration of 20 percent and preparations are under way to install many more.

The report comes only a few days after a senior IAEA expert team returned from Iran after the two-day second round of talks with Iranian officials. The meeting followed previous talks held in late January.

During both the first and second meetings, the IAEA team requested access to the military site at Parchin, but Iran did not grant the permission," according to an IAEA statement.

The IAEA head Yukiya Amano called Iran's refusal to permit the visit to Parchin as "disappointing."

The agency suspects the Parchin site is where Iran's suspicious nuclear activities are carried out.

Iran's envoy to the IAEA Ali Asghar Soltanieh has told the press that access to Parchin can only be granted when Iranian officials and the agency can agree on the conditions under which such a visit would take place.

Based on a previous IAEA report last November, the board of the UN nuclear watchdog had adopted a resolution on Iran's nuclear program, calling for intensified dialogue between the agency and Iran to find solutions to unresolved issues.

The November report, which alleges that Iran has engaged in nuclear bomb-related activities, was rejected by Tehran. The country called the report "imbalanced, non-professional and politically-motivated."