Iran has said it will allow UN inspectors access to a secret military complex where the UN nuclear agency suspects atomic work has been carried out.
A statement issued by Tehran's permanent envoy to the International Atomic Energy Agency said officials will allow UN staff to visit the Parchin complex, south east of Tehran, in a gesture of goodwill.
The statement, issued through the official ISNA news agency, said the visit requires an agreement on guidelines for the inspection.
Inspecting Parchin was a key request made by senior IAEA teams that visited Tehran in January and February.
Iran rebuffed those demands at the time, as well as attempts by the nuclear agency's team to question Iranian officials and secure other information linked to the allegations of secret weapons work.
The latest development comes a day after IAEA chief Yukiya Amano expressed growing concern that there was new activity at Parchin.
Mr Amano did not specify whether he believed the activity was linked to suspected new weapons experiments or attempts to clean up previous alleged work.
The Parchin complex has been often mentioned in the West as a suspected base for secret nuclear experiments - a claim Iran consistently denies.
IAEA inspectors visited the site in 2005, but only saw one of four areas on the grounds and reported no unusual activities.
Last year, IAEA's report said there were indications Tehran had conducted high-explosives testing to set off a nuclear charge at Parchin.
Iran denied the atomic activity and insisted that any decision to open the site rested with the armed forces since it was a military, not nuclear, facility.
Speaking on Monday in Vienna, Austria, Mr Amano said the suspicions of "activities ... ongoing at the Parchin site" means "going there sooner is better than later" for IAEA inspectors seeking to probe suspicions that Iran has been working secretly to develop nuclear arms.
"We have our credible information that indicates that Iran engaged in activities relevant to the development of nuclear explosive devices," he told reporters outside a 35-nation IAEA board meeting in Vienna, describing his sources as "old information and new information".
Iran denies any intention of possessing nuclear weapons and says all of its atomic activities are peaceful, but the IAEA says it has intelligence-based suspicions that may not be the case based on thousands of pages of documentation.
"Given that Parchin is a military site, access to this facility is a time-consuming process and it can't be visited repeatedly," ISNA quoted the Iranian statement as saying. It added that following repeated IAEA demands, "permission will be granted for access once more".
The statement added that Tehran and the IAEA need to agree on "modalities" before the visit can take place.
There was no immediate comment at the IAEA headquarters in Vienna on Tehran's decision. It comes as fears grow that Israel's air force may soon strike Iran in an attempt to destroy its nuclear facilities.
President Barack Obama met Benjamin Netanyahu in Washington yesterday and told the Israeli prime minister that the United States "will always have Israel's back," but that diplomacy is the best way to resolve the crisis over potential Iranian nuclear weapons.
Later the EU said the five permanent members of the UN Security Council - the US, the UK, China, France and Russia - and Germany had accepted an offer to resume talks with Tehran on the nuclear issue.
EU foreign policy chief Catherine Ashton responded on Tuesday to a February letter from Iranian nuclear negotiator Saeed Jalili in which he proposed new discussions. Baroness Ashton is the point of contact for the group.
She said in a statement that the EU hoped Iran "will now enter into a sustained process of constructive dialogue which will deliver real progress in resolving the international community's long-standing concerns on its nuclear programme."
The time and venue of the talks have not yet been determined.