VIENNA (Reuters) - U.N. nuclear inspectors returned from Iran on Friday without securing an agreement to investigate suspected atom bomb research, they said after two days of talks in Tehran.

Herman Nackaerts, deputy director general of the International Atomic Energy Agency (IAEA), said his inspectors had not been granted the access they have long sought to a military site and that further talks had been scheduled for February 12.

The absence of a breakthrough was a new setback for diplomatic efforts to allay international concerns over Tehran's atomic ambitions and avert the threat of a new Middle East war.

The IAEA, whose mission is to prevent the spread of nuclear weapons, has been trying for a year to negotiate a so-called structured approach with Iran giving the inspectors access to sites, officials and documents for their long-stalled inquiry.

At the centre of its concerns, the IAEA wants access to the Parchin military complex southeast of Tehran where it believes explosives tests relevant for nuclear weapons development may have taken place, something Iran denies.

"We had two days of intensive discussions. Differences remain so we could not finalize the structured approach to resolve the outstanding issues regarding possible military dimensions of Iran's nuclear program," Nackaerts told reporters at Vienna airport.

(Reporting by Fredrik Dahl)