US Secretary of State John Kerry says progress has been made in tough talks with Iran on its nuclear program, praising what he called the "constructive" atmosphere.
"I think we have resolved some of the things that were outstanding and we've made some progress," he said on Friday, speaking to reporters as he met with his team of experts in Vienna.
Meetings have been happening all day, Kerry said, adding: "We have a couple of different lines of discussion that are going on right now."
"The atmosphere is very constructive," he told the reporters who travelled with him from Washington.
"We still have a couple of very difficult issues, and we'll be sitting down to discuss those in the very near term - this evening and into tomorrow."
The talks are now heading into their third weekend in Vienna as global powers and Tehran seek to bridge the final gaps blocking a long-sought deal.
They are seeking to end a 13-year standoff with Iran over its suspect nuclear program with a deal which will block its pathways to developing a nuclear bomb in exchange for lifting biting global sanctions.
The terms of a 2013 interim accord under which Iran has suspended much of its uranium enrichment in return for some sanctions relief were extended to Monday in a bid to overcome a deadlock.
France's Laurent Fabius and British Foreign Secretary Philip Hammond are also due to return to the Austrian capital Saturday in the hopes of advancing the deal.