Iran is to allow the UN nuclear watchdog to carry out inspections at one of its most secret military sites.
Inspectors from the International Atomic Energy Agency (IAEA) are on standby to visit the plant, at Parchin.
US officials have accused Iran of using a civilian nuclear programme as front to develop atomic weapons.
Iran has consistently denied the claim, but an IAEA report published last October expressed concern over possible "dual-use" of nuclear technology.
The head of the IAEA, Mohammed ElBaradei, said the organisation will send inspectors to Parchin "within days or weeks".
US officials have alleged that a secret annex at the Parchin plant, 30 km (20 miles) south-west of Tehran, could be used for research into high explosives.
The IAEA is co-ordinating details of the visit with the Iranian authorities, Mr ElBaradei said.
Sensitive evidence
Iran has been under investigation for allegedly pursuing a programme of "weaponisation" while publicly claiming to pursue a civilian nuclear programme.
In November the US failed in a bid to have the IAEA refer Iran to the UN Security Council over alleged breaches of IAEA resolutions on weapons development.
Instead Iran offered to halt the development of centrifuges designed to enrich uranium - a key stage in "weaponisation".
European nations, led by Britain, France and Germany, have used diplomacy to convince Iran to co-operate with the international community.
The US maintains that Iran is developing weapons, but says that evidence it holds is sensitive and hard to verify.
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