Iran's president spoke with the leaders of France, Britain, China and Russia on Thursday in an apparent effort to break an impasse to a nuclear deal between Tehran and major world powers.

He also raised the Saudi-led military operation against Iranian-backed Houthi fighters in Yemen, as did U.S. Secretary of State John Kerry ahead of nuclear negotiations in Switzerland with Tehran's Foreign Minister Mohammad Javad Zarif.

The United States is pushing for a nuclear deal between Iran and major powers before a March 31 deadline, and officials close to the talks said some kind of preliminary agreement was possible.

However, a senior British diplomat acknowledged: "There are still important issues where no agreement has so far been possible.

"Our task, therefore, for the next few days is to see if we can bridge the gaps and arrive at a political framework which could then be turned into an agreement," the diplomat told reporters on the sidelines of negotiations in Lausanne, Switzerland.

The six powers, which include Germany and the United States, hope to reach a comprehensive nuclear agreement by June 30.

Western powers fear Iran wants to build nuclear bombs, though Tehran says its atomic research is for peaceful purposes. The powers hope to persuade Iran to scale back its nuclear activity in return for the removal of economic sanctions.

Israel is believed to have the Middle East's only nuclear arsenal and has in the past threatened Iran with military attack.