The final round of the Iran nuclear talks began Saturday with Secretary of State John F. Kerry and Iranian Foreign Minister Mohammad Javad Zarif cautioning that a lot of hard work lies ahead to reach a deal in the dwindling days before a deadline.

The two men and their negotiating teams talked with one another for nearly three hours in a small meeting room at the posh Palais Coburg hotel.

Asked whether they were optimistic that they would be successful this week, Kerry said, “Well, I think it’s fair to say that we’re hopeful. We have a lot of hard work to do. There are some very tough issues, and I think we all look forward to getting down to the final efforts here to see whether or not a deal is possible. I think everybody would like to see an agreement. But we have to work through some difficult issues.”

Zarif concurred but made it clear that large differences remain on several issues, known to include the pace of sanctions relief and what kind of access will be provided to international inspectors monitoring Iranian compliance.

“I agree,” Zarif said. “Maybe not on the issues. But on the fact that we need to work really hard in order to be able to make progress and move forward. We are determined to do everything we can to be able to make this important milestone. Of course, that depends on a lot of things, and we’re going to work on it.”

After the meeting with Zarif, Kerry was to meet later in the day with French Foreign Minister Laurent Fabius.

The negotiators are facing a deadline Tuesday for a final deal that aims to limit Iran’s capacity to produce nuclear weapons and to ease sanctions that have been in place for decades.

A framework agreement, reached April 2, outlines the basic elements of a deal, and it was hailed as a breakthrough. Since then, though, negotiators have been stuck on the precise details.

Complicating matters, Iran’s supreme leader, Ayatollah Ali Khamenei, recently listed seven “major red lines” that appear to backtrack on what Iran is willing to concede.

Khamenei insists that economic and banking sanctions must be lifted the day an agreement is concluded, not in phases as Iran is seen as living up to its commitments. And he has said that inspectors from the International Atomic Energy Agency will not have access to Iran’s nuclear scientists or military sites, hindering investigation of surreptitious work that could be used to build nuclear weapons.

Intense talks have been going on for more than a year and a half. In this final lap, the talks are hampered by the absence of Ali Akbar Salehi, the head of Iran’s atomic energy agency. Salehi was instrumental in talking with U.S. Energy Secretary Ernest Moniz, both MIT-educated nuclear physicists who worked on the highly complex technical details needed to satisfy the concerns of Iran and the six world powers that are also negotiating. Salehi is recovering from surgery last month to repair a perforated bowel. He is expected to call in to the talks.

This is Kerry’s first trip overseas since he broke a leg in a bicycle accident May 31 after a day of talks with Zarif in Geneva. He is able to move about on crutches — or “sticks,” as he calls them — and was raised by a mechanized lift to the aircraft that brought him to Vienna on Friday.

In addition, Zarif is known to have chronic back problems exacerbated by the long negotiating sessions in which they sit for hours at a time.