Officials from Iran and the United Nations' nuclear agency have begun a new round of talks in Vienna about Tehran's nuclear programme.
The agency, the IAEA, says it wants Iran to address suspicions about some possible military applications.
The agency's chief inspector said it was important that Iran allow the IAEA access to people, documents and sites.
Their last meeting, in February, failed to reach agreement. Iran says its nuclear programme is purely peaceful.
Speaking before Monday's talks the IAEA Deputy Director General Herman Nackaerts urged Iran to allow his team to visit a suspect military site at Parchin.
Monday's session lasted about five hours, but ended with officials refusing to make any comment about how it had progressed.
It is thought that the talks will continue on Tuesday.
This meeting comes a week before the next round of talks between six world powers and Iran are scheduled to begin in Baghdad.
The last series of full international talks with Iran broke down in early 2011.
Since then, the IAEA has expressed concern that Iran has failed to co-operate with its inspectors and has carried out activities "relevant to the development of a nuclear explosive device".
Israel, which believes a nuclear-capable Iran would be a direct threat to its security, has warned that time is running out to prevent that outcome.
Prime Minister Benjamin Netanyahu has hinted that Israel is prepared to attack Iran's nuclear facilities if diplomacy fails.
However President Obama has warned against "loose talk of war", while stressing that all options remain open.