Six world powers have agreed on a way to restore U.N. sanctions on Iran if the country breaks the terms of a future nuclear deal, clearing a major obstacle to an accord ahead of a June 30 deadline, Western officials told Reuters.

The new understanding on a U.N. sanctions “snapback” among the six powers – the United States, Britain, France, Germany, Russia and China – brings them closer to a possible deal with Iran, though other hurdles remain, including ensuring United Nations access to Iranian military sites.

The six powers and Iran struck an interim agreement on April 2 ahead of a possible final deal that would aim to block an Iranian path to a nuclear bomb in exchange for lifting sanctions. But the timing of sanctions relief, access and verification of compliance and a mechanism for restoring sanctions if Iran broke its commitments were among the most difficult topics left for further negotiations.