Even if an outline agreement is reached in Lausanne, the most difficult issues will still have to be resolved before the June 30th deadline for a final accord.
The Iranian President Hassan Rouhani has said his country will honour its part of the framework deal reached on its nuclear programme, as long as others do the same.
The two sides agreed to a framework in April, but still need to work out details such as how quickly to lift the sanctions that have hurt Iran's economy.
On Tuesday, after nearly two weeks of difficult talks, Ukrainian and U.S. officials said they had reached agreement on a framework for sharing revenue from Ukraine's critical minerals.
The initial framework deal doesn't specify what it covers, mentioning only relevant resource assets that will be further described in another agreement.