The UN atomic watchdog chief said Friday, after a crucial visit to Iran, that both sides have a "better understanding" in some areas but that more work is required.
International Atomic Energy Agency (IAEA) head Yukiya Amano visited Tehran on Thursday with the aim of clearing up one of the thorniest issues in talks in Vienna towards a historic nuclear deal between Iran and major powers.
"I believe that both sides have a better understanding on some ways forward, though more work will be needed," Amano said in a statement.
The issue concerns a stalled IAEA probe into allegations that before 2003, and possibly since, Iran conducted research work into developing nuclear weapons.
Iran rejects the allegations, saying they are based on bogus intelligence provided to a gullible and partial IAEA by the likes of the CIA and Israel's Mossad.
The talks between Iran and the P5+1 -- the United States, Russia, China, Britain, France and Germany -- are focused more on the future capacities of Iran's nuclear programme.
But the major powers say that advancing the IAEA's investigation, which could require the agency to visit military sites, is vital for the wider accord, which they aim to close by Tuesday.