PARIS, June 20 (Reuters) - Parties negotiating a revival of the Iran nuclear deal will hold a formal meeting in Vienna on Sunday, the European Union said on Saturday.

Iran and six world powers have been negotiating in Vienna since April to work out steps for Washington and Tehran to take. The United States withdrew in 2018 from the pact, under which Iran accepted curbs on its nuclear programme in exchange for a lifting of many foreign sanctions against it.

Sunday's formal meeting comes more than a week after this round of talks resumed and is an indication that the talks are likely to be adjourned.

Officials over the week have indicated that differences remain on key issues.

"The Joint Commission of #JCPOA will meet on Sunday, June 20," Mikhail Ulyanov Russia's envoy to the talks said on Twitter.

"It will decide on the way ahead at the #ViennaTalks. An agreement on restoration of the nuclear deal is within reach but is not finalised yet."

The remaining parties to the deal - Iran, Russia, China, France, Britain, Germany and the European Union - meet in the basement of a luxury hotel.

The U.S. delegation to the talks is based in a hotel across the street as Iran refuses face-to-face meetings, leaving the other delegations and EU to work as go-betweens.

Since former U.S. President Donald Trump pulled out of the deal and reimposed sanctions on Iran, Tehran has embarked on counter measures, including rebuilding stockpiles of enriched uranium, a potential pathway to nuclear bombs.