Access to the Parchin military complex, just outside Tehran, is a key element in the negotiations.

CNN: Official: Iran, nuclear watchdog group deal close

Iran denies that Parchin has any role in its nuclear program, which it insists is for peaceful purposes.

CNN: STORY HIGHLIGHTS

They called on Iran to co-operate fully with UN inspectors and allow them to visit the Parchin military site.

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The International Atomic Energy Agency (IAEA) has previously said it suspects the Parchin site may be being used for nuclear weapons-related testing.

BBC: Iran nuclear: Israel's Netanyahu warns on attack timing

In its last report in November 2012, the agency expressed particular concern about activities at a facility in Parchin, just 20km (13 miles) from Tehran.

BBC: Iran's Khamenei rejects US nuclear talks offer

The chief of the United Nations nuclear watchdog has called for access to Iran's Parchin military site "without further delay" and without waiting for negotiations to make progress.

BBC: IAEA demands Iran nuclear access to Parchin site

Iran had earlier said it was prepared under certain conditions to grant inspectors from the UN's International Atomic Energy Agency (IAEA) access to the key military site of Parchin.

BBC: Fresh Iran nuclear talks agreed with world powers - EU

Iran's liaison to IAEA, Ali Asghar Soltanieh, characterized the talks as constructive, according to state-run news agency IRNA. The report stopped short of saying whether the talks included discussion about investigators getting access to Parchin, which was requested at a meeting in November in Vienna, Austria.

CNN: Official: Iran, nuclear watchdog group deal close

Last month he sent a delegation to Tehran in the latest of a series of talks to negotiate a possible resumption of site visits, but the head of Iran's atomic energy programme Fereydun Abbasi-Davani said visits to Parchin or any other site were "not on the agenda".

BBC: IAEA demands Iran nuclear access to Parchin site