The U.S. and Iranian foreign ministers will meet June 27 to try to settle major differences over an agreement providing Iran with sanctions relief in exchange for curbs on its nuclear activities.
U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif will meet in Vienna ahead of a self-imposed deadline of June 30, which officials expect to slip by at least two or three days.
Significant gaps remain over the sequencing of economic sanctions relief and the nature of monitoring mechanisms to ensure Tehran does not cheat on any agreement.
"The next few days will be extremely difficult," a senior Western diplomat said.
Senior Iranian negotiator Abbas Araqchi said the negotiations have been "slow and difficult."
Foreign ministers from Britain, China, France, Germany, and Russia are expected to "check in" as well.
They must settle disputes over the speed and scope of lifting sanctions, how Iran will reduce its stockpiles of low-enriched uranium, the future extent of Tehran's enrichment-centrifuge research, and access for United Nations inspectors to Iranian nuclear scientists and military sites.
Also outstanding are questions about the military dimensions of Iran's past nuclear work.