Top diplomats from Iran and the U.S. began a new round of marathon talks on a nuclear deal late Monday, as Israeli leader Benjamin Netanyahu warned the emerging accord could threaten his country's survival.

U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif met in a Swiss lakeside hotel for a series of sessions which are scheduled to stretch into Wednesday afternoon.

They were accompanied by Energy Secretary Ernest Moniz and Iranian nuclear chief Ali Akbar Salehi, who according to the Iranian news agency IRNA had first met earlier for about 90 minutes.

Iranian negotiators Abbas Araghchi and Majid Takht Ravanchi had also held talks with U.S. Under Secretary Wendy Sherman as well as the EU's deputy foreign policy chief Helga Schmid, IRNA added.

The pace and intensity of the negotiations to hammer out a deal to rein in Iran's suspected nuclear programme in exchange for sanctions relief have gathered pace as a March 31 deadline for a political accord nears.

"We are all focused simultaneously on the need to elicit from Iran answers to questions about their nuclear programme -- not just answers for today, but answers that are capable of lasting well into the future," Kerry told reporters in Geneva.

He stressed global powers, grouped under the P5+1, were not seeking "a deal at any cost" but to ensure that the "four pathways to a nuclear bomb have been closed off."

"We hope we can get there, but there is no guarantee," Kerry added.

Kerry's first round of talks with Zarif on Monday ended after about 50 minutes. It remained unclear if they would hold more meetings later in the evening.

As a deal appears within grasp, the U.S. administration has clashed with Israel, with Netanyahu saying he would address Congress on Tuesday "to speak up about a potential deal with Iran that could threaten the survival of Israel."

"Israel and the United States agree that Iran shouldn't have nuclear weapons. But we disagree on the best way to prevent them from developing those weapons," he told a powerful pro-Israel lobby in Washington.

Kerry and Zarif have met frequently in past weeks in European cities, hoping to pin down a deal which has eluded the international community for more than a decade. (AFP)