The meeting between Kerry and Mohammad Javad Zarif comes with diplomats on both sides say that the accord curbing Iran’s nuclear activities in exchange for sanctions relief is far from done.

Iran will reach a nuclear deal with world powers so long as the other side does not make excessive demands, Zarif was quoted as saying on Saturday as he arrived in Vienna for the final stage of the talks.

“If the other side … takes positive steps and does not make excessive demands, we will certainly reach a deal that benefits everyone,” Zarif was quoted as saying by state news agency IRNA.

Kerry arrived in Vienna late Friday seeking to wrap up a nearly two-year quest for a historic deal to curb Iran’s nuclear ambitions.

Undaunted by having to walk with crutches after breaking his leg last month, Kerry and his team were preparing to return to the negotiations on Saturday with a deadline for a deal just four days away.

Significant gaps remain, notably over the sequencing of economic sanctions relief for Iran 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 on condition of anonymity, adding that the talks may slide at least two or three days past the deadline.

Senior Iranian negotiator Abbas Araqchi told Iranian reporters that the current negotiations were a “slow and difficult process.”

In addition to Iran and the United States, the talks include Britain, China, France, Germany and Russia. Foreign ministers from all the nations are expected in Vienna in the coming days to “check in” on the progress of negotiations.

Officials close to the talks say they have yet to agree on 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 development program, and access for United Nations inspectors to military and other sites, as well as U.N. access to Iranian nuclear scientists.

Another outstanding issue is what Iran may be required to do to address questions about the potential military dimensions (PMD) of its past nuclear work.

Iran wants sanctions lifted immediately, though diplomats say they will be eased gradually in accordance with a schedule and only after confirmation that Iran has met its commitments to curb its nuclear program.

Iran rejects allegations from Western powers and their allies that it is seeking the capability to produce nuclear weapons under cover of a civilian atomic energy program.