UN nuclear inspectors have been stopped from visiting a key military site in Iran.
The International Atomic Energy Agency (IAEA) has said its officials are now leaving the country after failing to reach a deal to inspect the Parchin military site, near Tehran.
"Intensive efforts were made to reach agreement on a document facilitating the clarification of unresolved issues in connection with Iran's nuclear programme," the IAEA said in a statement.
"Unfortunately, agreement was not reached on this document."
The team requested access both during this visit and during a first trip in January to the Parchin site where it believes explosives testing is being carried out.
"It is disappointing that Iran did not accept our request to visit Parchin during the first or second meetings," IAEA Director General Yukiya Amano said in the statement.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, was quoted by the Iranian news agency ISNA as saying the talks had been intensive and covered "cooperation and mutual understanding between Iran and the IAEA".
"These negotiations will continue in the future," he said.
The visit was aimed at clarifying all "outstanding substantive issues" surrounding Tehran's nuclear programme, in particular what the IAEA called "possible military dimensions".
The trip was also seen as an important precursor to a possible resumption of talks between Iran and the US, China, Russia, France, Britain and Germany.
Last November the IAEA said that Iran had carried out activities in a number of areas "relevant to producing" a nuclear weapon.
Since then the US and the European Union have ramped up sanctions on Iran's oil sector.
In retaliation Iran has threatened to close the Strait of Hormuz and announced the halt of oil sales to Britain and France.