TEHERAN (BLOOMBERG) - Negotiations between the US and Iran to revive the 2015 nuclear deal will restart in the "coming days", according to the European Union's chief diplomat.

"The coming days mean coming days", Josep Borrell said on Saturday (June 25) during a press conference in Teheran with his Iranian counterpart. "I mean quickly, immediately."

The talks will be indirect, with the EU acting as a mediator, and take place in a Persian Gulf country, Iranian media quoted Borrell as saying later the same day.

Negotiations between Iran and world powers stalled in March after about a year of discussions in Vienna, where the original accord was sealed. The pact, known as the Joint Comprehensive Plan of Action, curbed Teheran's atomic activities in exchange for the easing of some economic sanctions, including on energy exports.

Then-president Donald Trump withdrew the US from the agreement in 2018. Reviving it may lead to more Iranian oil sales and help lower crude prices, which have surged 45 per cent this year to around US$110 (S$152) a barrel.

The talks "have to be finished, three months have passed", Borrell said.

A key sticking point has been Iran's demand for Washington to stop classifying the Islamic Revolutionary Guard Corps, a military organisation blamed for numerous attacks on the US and its allies, as a terrorist organisation.

Iran's Foreign Minister Hossein Amirabdollahian reiterated his call for US President Joe Biden to take a "realistic and fair" approach to help reach an agreement.

Many members of Congress and Israel oppose a return of the JCPOA, arguing it could hand Iran an oil windfall and lead it to rearm its proxies in the Middle East.

Still, many world powers say a deal is the best way to prevent Iran from obtaining a nuclear weapon. The country has increased its enrichment of uranium since Trump exited the JCPOA.

Iran has always maintained its atomic programme is peaceful.