Iranian President Hassan Rouhani speaks in a ceremony at Imam Khomeini International Airport. File picture: Iranian Presidency Office via AP

Vienna - Senior diplomats from five major powers are meeting their Iranian counterparts in Vienna on Friday in an attempt to stop Tehran from abandoning their joint nuclear deal.

The United States walked away from the 2015 pact last year and revived sanctions that target the Islamic Republic's oil exports.

Tensions between Washington and Tehran came to a head in recent days, when President Donald Trump came close to ordering an airstrike in retaliation against Iran's downing of a US drone.

Iran said in May that it would gradually exit the 2015 deal, by breaking certain limits on uranium enrichment this week and in early July that were designed to prevent the development of a nuclear warhead.

However, an Iranian official said on the eve of the Vienna meeting that the first of the two limits, which regulates how much uranium can be stocked, has not yet been surpassed.

Speaking on condition of anonymity, the official said that "there is always a last-minute chance" for diplomacy.

Iranian Deputy Foreign Minister Abbas Araqhchi told Iranian reporters in Vienna on Thursday that Britain, Germany and France should present concrete plans to mitigate US sanctions.

"Otherwise we will definitely reduce our commitments under the nuclear deal," he warned.

On Friday, European powers are expected to promote their so-called Instex scheme, a planned barter system for European-Iranian trade that is meant to avoid US sanctions.

However, Instex is unlikely to revive Iran's oil exports as companies with business in the US fear that any contracts with Iran would make them a target of Washington's punitive measures.

dpa