Iranian Foreign Minister Javad Zarif (R) listens to questions from journalists as he stands next to Iran's chief nuclear negotiator Abbas Araghchi (L) and Hossein Fereydoon (2nd L), brother and close aide to President Hassan Rouhani, on the balcony of Palais Coburg, the venue for nuclear talks in Vienna, Austria, July 10, 2015. [Photo/Agencies]

VIENNA - Big progress was made in certain issue of the nuclear talks, while work is still needed in a few remaining issues, a diplomatic source told Xinhua on Friday.

The western states and Tehran have almost agreed on the clarification of the alleged Iran's past nuclear weapon program, the so called possible military dimensions (PMDs) issues and big progress was also made over capping Iran's nuclear capability in the deal, said the source.

UN nuclear watchdog accused that Tehran might have carried out nuclear bomb research program, PMDs, under the disguise of its civilian program, while Iran rejects the allegation. Iran and the agency agreed to resolve the PMDs issues but the progress is slow.

Western states decided to extend the Geneva interim deal till next Monday, keeping suspension of some sanctions to be imposed on Tehran, making the last diplomatic efforts to resolve the disputed Iran nuclear issue.

The big progress in capping Iran's nuclear capacity in the deal indicates there could be a breakthrough on defining Iran's advanced centrifuge research in the future.