Representatives from Iran and the U.N.'s nuclear watchdog have wrapped up another round of talks, with no agreement over access to a disputed nuclear site.
The two sides met Friday in Vienna to discuss access to Iran's Parchin facility, a military installation southeast of Tehran. The West suspects the site is related to possible nuclear weapons development.
After the talks, International Atomic Energy Agency, IAEA, chief inspector Herman Nackaerts told journalists the agency had hoped to resolve remaining differences.
"Discussions today were intensive but important differences remained between Iran and the Agency that prevented an agreement," he said.
Nackaerts said there were no immediate plans for another meeting.
Meanwhile, Iranian envoy Ali Asghar Soltanieh said the two sides made headway.
"We have to say that undoubtedly some progress in removing some ambiguities and differences has been made but still, as it was said, there are some differences because it is a very complex issue, and issues related to national security of a member state are something very delicate, but I have to say we are moving forward," Soltanieh said.
The two sides have been in periodic negotiations for months over access to the Parchin site.
The International Atomic Energy Agency says Parchin may have been a testing ground for the making of a nuclear warhead.
News reports quoting comments from the IAEA in recent days express fears that Iran is ramping up its work on developing nuclear arms by installing new uranium enrichment centrifuges at another site, the Fordow underground facility.
Iran says its nuclear ambitions are peaceful.
The talks come as representatives of nearly 120 nations, including dozens of heads of state, are to convene in Tehran on Sunday for the summit of the Non-Aligned Movement.
The Non-Aligned Movement is an organization formed during the Cold War to provide a forum for countries that were not officially allied with either the United States or the Soviet Union.
The U.S. State Department says Iran will try to manipulate the Non-Aligned Movement at the summit and try to divert attention from its defiance of several U.N. Security Council resolutions and international sanctions over its disputed nuclear programs.