The U.S. Secretary of State, as well as the Chinese, German, French, and Iranian foreign ministers were in the Swiss resort of Lausanne on March 29 for the negotiations.

They were joined by senior representatives from Britain and Russia.

The delegations are working on an initial agreement aimed at curbing Tehran's nuclear activities in exchange of sanctions relief by a March 31 deadline.

That accord is meant to pave the way for a final agreement by the end of June.

The five permanent members of the UN Security Council -- Britain, China, France, Russia, and the United States -- plus Germany want to ensure that Iran cannot develop nuclear weapons.

Tehran denies it is secretly developing a nuclear weapon, saying it only wants to develop civilian nuclear power for domestic needs.

Officials attending the talks reportedly said the parties had offered mutual compromises in an attempt to break the deadlock.

The AFP and Reuters news agencies quoted unidentified diplomatic sources as saying Tehran indicated a willingness to slash the number of its centrifuge machines and sending most of its stockpile of nuclear material abroad.

They said Western powers were meanwhile considering the idea of allowing Iran to conduct limited, closely monitored enrichment-related work for medical purposes.

The U.S. State Department said Secretary of State John Kerry had cancelled plans to return to the United States for an event honoring his late Senate colleague Edward Kennedy in order to remain at the talks in Switzerland.

Reports said German Foreign Minister Frank-Walter Steinmeier and his French counterpart, Laurent Fabius, delayed a planned trip to Kazakhstan in order to focus on the negotiations.

On March 28, Iranian Foreign Minister Mohammad Javad Zarif said in Lausanne that progress had been made.

"I think we can in fact make the necessary progress to be able to resolve all the issues and start writing them down in a text that will become the final agreement once it's done," Zarif told reporters following separate meetings with Fabius and Steinmeier.

Israel, which is not a party to the talks, has again warned of a nuclear deal with Iran.

Addressing a cabinet meeting on March 29, Prime Minister Benjamin Netanyahu said, "this deal, as it appears to be emerging, bears out all our fears, and even more than that."

Citing advances by Iran-backed Shi'ite rebels in Yemen, he accused Iran of trying to "conquer the Middle East" while pursuing nuclearization.

"The Iran-Lausanne-Yemen axis is very dangerous for humanity and must be stopped," Netanyahu added.