France's foreign minister said on Wednesday that Iran still had to make more of an effort to reach a preliminary political accord, but that Tehran and the six powers were in the final and most difficult stages.

"We are a few meters from the finishing line, but it's always the last meters that are the most difficult. We will try and cross them," Laurent Fabius said on his return to the talks in Lausanne, Switzerland.

"It's not done yet. We want a robust and verifiable agreement and there are still points where there needs to be progress, especially on the Iranian side," he said.