Yukiya Amano, head of the International Atomic Energy Agency (IAEA), spoke a day after holding rare talks in Tehran and a day before Iran and six world powers will hold broader negotiations on the extent of Tehran's nuclear programme.

"(A) decision was made to conclude and sign the agreement ... I can say it will be signed quite soon," Amano told reporters at Vienna airport after returning from Tehran.

Amano, who had been looking for a deal giving his inspectors a freer hand to investigate suspected atomic bomb research in Iran, described the outcome of his meetings in Iran as an "important development".

He said "some differences" remained but that Iranian chief nuclear negotiator Saeed Jalili had told him these would not pose an obstacle to an agreement.

Jalili on Wednesday will sit down in Baghdad with senior officials from the six world powers involved in efforts to resolve the Iranian nuclear stand-off peacefully.

"We understand each other's position better," Amano said about his talks with Jalili and other Iranian officials.

He said he had raised the issue of access to the Parchin military site - an IAEA priority in its inquiry - and that this would be addressed as part of the agreement's implementation. (Reporting by Fredrik Dahl; Editing by Mark Heinrich)