LAUSANNE, Switzerland - A joint statement was being finalized on Thursday at talks between Iran and world powers over Tehran's nuclear program, but officials stopped short of suggesting that a deal had been reached.

A US official and a Western diplomat said the joint statement was being drafted to be issued by Iran's Foreign Minister Javad Zarif and EU foreign police chief Federica Mogherini.

An Iranian official also confirmed Zarif and Mogherini would issue a joint statement, suggesting it might fall short of an announcement of a deal.

"This is only a statement," the Iranian official said.

Mogherini has acted as a coordinator for the six powers - Britain, China, France, Germany, Russia and the United States - negotiating with Tehran to prevent it from acquiring the means to develop a nuclear bomb.

The diplomat said a full session between representatives of six powers and Iran was about to start at the talks in Lausanne, Switzerland.

Neither official would confirm that a deal had been reached at the marathon talks, which aim to reach a preliminary political agreement before a final deal is reached by June 30.

The Swiss talks were due to end by Tuesday but have been extended for two days, with sides saying they were making progress but having trouble agreeing on details.