In one day talks with Iranian officials Friday, inspectors from the UN nuclear watchdog failed to gain access to a key nuclear research site, where they suspect advanced tests related to atomic weapons were carried out and later covered up, dpa reported.
Iran's envoy in Vienna, Ali Asghar Soltanieh, said: "There was undoubtedly some progress in removing some differences, because it is a very complex issue. We are moving forward - it is an indication that we can work with the agency closely."
The International Atomic Energy Agency (IAEA) believes that Iran has used the Parchin plant, south of Tehran, to carry out nuclear tests.
"We are here today to continue our discussions with Iran to seek agreement on a step-by-step approach to solve all the outstanding issues," IAEA chief inspector Herman Nackaerts said in Vienna before entering the Iranian embassy.
"Of course, we will also ask Iran where they are with their responses to our request for access to Parchin."
After the meeting, Nackaerts said: "Discussions today were intensive, but important differences remain."
According to the IAEA, satellite imagery from recent days indicate that several buildings have been torn down at the site and material transported away, suggesting Iran is attempting to erase evidence of illicit nuclear activities.
Israeli Prime Minister Benjamin Netanyahu charged Friday that Iran has made increased progress towards developing a nuclear arsenal, despite international attempts to get it to halt its atomic programme.
"Only yesterday we received additional proof that Iran is continuing accelerated progress towards achieving nuclear weapons and is totally ignoring international demands," a statement from Netanyahu's office said.
Israeli officials, primarily Netanyahu and Defence Minister Ehud Barak, have amplified their rhetoric against Iran in recent weeks, heightening speculation that an Israeli attack on Iran is only a matter of months, or even weeks.
Israel regards Iran's suspected nuclear weapons programme as a prime existential threat, especially given the repeated statements by Iranian leaders that the Jewish state should be erased off the map.
Iran however insists its nuclear programme is peaceful, and has held inconclusive nuclear talks with China, Russia, France, Britain, the United States and Germany - who are pressing it to halt nuclear enrichment activities.
The IAEA's quarterly report on Iran is due next week. The issue is also expected to be raised at the next meeting of the body's governing board, set for September 10-14.