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

The two men and the negotiating teams they lead started talking shortly after noon in a small meeting room at the posh Coburg Palace Hotel.

Asked whether they were optimistic 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.�

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

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 both Iran and the six world powers negotiating with them. 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 get about on crutches � or �sticks,� as he calls them � and was raised by a mechanized lift.

This article was written by Carol Morello for The Washington Post on June 27, 2015. Carol Morello is the diplomatic correspondent for The Washington Post, covering the State Department.