Top diplomats from Iran and the US launched 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.

US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif met twice in a Swiss lakeside hotel at the start of a series of sessions which are scheduled to stretch into Wednesday afternoon.

They were accompanied by US 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 also held talks with US 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 that global powers, grouped under the P5+1, were not seeking "a deal at any cost" but wanted 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 and Zarif first met for about 50 minutes Monday evening, and after a short break, held a second 25-minute meeting before calling it a night.

They are due to resume their talks early Tuesday, for what US officials have promised would likely be "a flowing meeting that runs into another."

As a deal appears within grasp, the US 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," Netanyahu 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.

UN nuclear watchdog pushes Iran for faster response
Vienna (AFP) March 2, 2015 - The head of the UN nuclear watchdog pressed Iran on Monday to speed up cooperation with the agency's probe into Tehran's atomic programme.

"This process cannot continue indefinitely. It is not an endless process," said Yukiya Amano, director-general of the International Atomic Energy Agency.

While Iran is negotiating limits on its programme with world powers, the IAEA has been pressing leaders in Tehran for years to address allegations that prior to 2003, and possibly since, they conducted research into developing nuclear weapons.

Iran has consistently rejected the claims, set out in a major IAEA report in 2011, as baseless.

However, Iran had agreed to answer agency inquiries about alleged explosive testing and research into nuclear bomb making by August, but the responses have yet to be given.

"The agency remains ready to accelerate the resolution of all outstanding issues," Amano said. However, "this requires increased cooperation by Iran and the timely provision of access to all relevant information."

Amano met with Iranian leaders in February, and last week Tehran's deputy foreign minister Abbas Araghchi pledged his country would "move faster and in a better sense" with the probe.

The P5+1 group of world powers have until March 31 to reach a framework for a deal, which would then be firmed up and officially signed on June 30.

US Secretary of State John Kerry is due to meet his Iranian counterpart Mohammad Javad Zarif in the Swiss lakeside town of Montreux later Monday for talks on the agreement.

US warns Israel PM not to 'betray' trust on nuclear deal
Geneva (AFP) March 2, 2015 - US Secretary of State John Kerry Monday warned Israel's prime minister against revealing details of an emerging Iran nuclear deal on the eve of the Israeli leader's speech to the US Congress.

While he did not mention Benjamin Netanyahu by name, Kerry told reporters in Geneva he was "concerned by reports" that "selective details" of the deal aimed at curbing Tehran's nuclear programme would be revealed in the coming days.

His deputy spokeswoman Marie Harf went further, saying discussing the contents of classified briefings by US officials to Israelis would "betray" America's trust.

"We've continuously provided detailed classified briefings to Israeli officials to keep them updated and to provide context for how we are approaching getting to a good deal," she told reporters in Washington.

"Any release of any kind of information like that would, of course, betray that trust."

The comments come after an Israeli official said the Jewish state knew about the emerging agreement and that the prime minister would elaborate in his congressional address.

Kerry launched a series of talks with his Iranian counterpart Mohammad Javad Zarif in the Swiss lakeside town of Montreux late Monday as they seek to pin down a deal by a March 31 deadline.

"The best way to deal with the question surrounding this nuclear programme is to find a comprehensive deal, but not a deal which comes at any cost," he told reporters.

"We have made some progress, but we still have a long way to go, and the clock is ticking."

The P5+1 group of world powers that are negotiating with Iran have only a few more weeks to reach a political framework for a deal, with the final technical details to be arrived at by June 30.

But Israel is worried the deal will ease sanctions on Tehran -- which is what Iran wants -- without applying sufficiently stringent safeguards to stop Iran acquiring enough fissile material to develop an atomic bomb.