VIENNA (Reuters) - No breakthrough was reached between the U.N. nuclear watchdog and Iran on unresolved questions around Tehran's nuclear program, the head of the International Atomic Energy Agency (IAEA) said on Friday after a visit to Tehran.

Tehran and world powers are pushing for a final deal at nuclear talks in Vienna, under which Iran would agree to scale back its atomic research - which it denies is aimed at developing a bomb - in exchange for a lifting of sanctions.

The IAEA will play a crucial role in monitoring and verifying any final deal. Iran has been stalling the agency's investigation, running parallel to the political talks, into possible military dimensions of Tehran's past nuclear work.

Yukiya Amano, the IAEA's director general, on Thursday met Iran's President Hassan Rouhani and the secretary of Iran's Supreme National Security Council, Ali Shamkhani, in Tehran.

"The purpose of the visit was to advance work towards the resolution of all outstanding issues regarding Iran's nuclear program, including clarification of possible military dimensions," Yukiya Amano said in a statement.

"I believe that both sides have a better understanding on some ways forward, though more work will be needed."

(Reporting By Shadia Nasralla; Editing by Andrew Heavens)