Six world powers called on Iran on Thursday to let international inspectors visit a military site where the UN nuclear watchdog says development work relevant to nuclear weapons may have taken place.
In a joint statement at a board meeting of the International Atomic Energy Agency, the powers also voiced "regret" about Iran's stepped-up campaign to enrich uranium - activity which can have both civilian and military purposes.
"We urge Iran to fulfill its undertaking to grant access to Parchin," the statement said, referring to the military facility southeast of Tehran. Iran refused access to the complex during two rounds of talks with a senior IAEA team earlier this year.
Western diplomats suspect the Islamic state may now be trying to clean up the site to remove evidence of research with nuclear applications before possibly allowing inspectors in.
The six powers handling the Iran nuclear issue are the United States, China, Russia, France, Germany and Britain.
An IAEA report last year revealed a trove of intelligence pointing to research activities in Iran of use in developing the means and technologies needed to assemble nuclear weapons, should it decide to do so.
One salient finding was information that Iran had built a large containment chamber at Parchin in which to conduct high-explosives tests that the IAEA said are "strong indicators of possible weapon development".
Iran, which says its nuclear program is purely for peaceful purposes, has dismissed intelligence reports suggesting it has a nuclear weapons agenda as forged and baseless.
It has suggested that the IAEA could get access to Parchin, but only after a broader deal is reached on how to address all outstanding issues between Tehran and the Vienna-based agency - an approach Western diplomats dismissed as a stalling tactic.
The world powers' statement, agreed after intensive discussions within the often disunited group, also voiced backing for efforts to find a diplomatic solution to the long-running row.
Israel and the United States have threatened Iran with military strikes as a last-ditch way to stop it getting nuclear weapons.
The European Union's foreign policy chief, who represents the powers in dealings with Iran, said on Tuesday they had accepted Iran's offer to return to talks after a standstill of a year that seen increasingly bellicose rhetoric.
"We ... reaffirm our continuing support for a diplomatic solution to the Iranian nuclear issue and readiness to restart dialogue with Iran," the powers said in their statement, read out by China's envoy to the IAEA at the closed-door meeting.
"We call on Iran to enter, without preconditions, into a sustained process of serious dialogue which will produce concrete results." Iran has refused at previous talks to negotiate on the future of its nuclear activity.
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