The US and Iran have suspended nuclear negotiations ahead of schedule, setting up new talks next week for a deal providing long-term assurance to the world that Iran cannot develop nuclear weapons.

The session in the Swiss city of Lausanne was interrupted on Friday - its sixth day - to enable members of the Iranian delegation to attend the funeral of their president's mother.

Those departing included Mohammad Javad Zarif, Iran's foreign minister; John Kerry, US secretary of state; and Hossein Fereydoon, a brother of Iranian President Hassan Rouhani.

The sides were close to an agreement, a senior Russian official said on Friday, when the talks between Kerry and the Iranian delegation were suspended. 

US President Barack Obama also spoke with his French counterpart Francois Hollande on Friday to discuss the talks, the White House National Security Council said on Twitter.

The leaders also discussed the situation in Ukraine, the White House said. 

Diplomats did not promise a breakthrough in the Lausanne talks, but tried to make as much progress as possible with a March 31 deadline for a framework accord looming.

Sergey Ryabkov, the top Russian negotiator, said that while some disputes remain, the US, Iran and five other world powers negotiating the deal are expected to "finish their main work" before the talks resume next week.

Iranian commitment

Ryabkov spoke shortly before Kerry's last meeting with Zarif on Friday. This week's discussions had been tentatively extended to go into Saturday.

Ryabkov's comments were consistent with those of other officials who told the AP earlier that the US and Iran were drafting elements of a deal that commits the Iranians to a 40 percent cut in the number of machines they could use to make an atomic bomb.

Diplomats have tried to make as much progress as possible in Lausanne with a March 31 deadline for a framework accord looming [AFP]

In return, Iran would get quick relief from some crippling economic sanctions and a partial lift of a UN embargo on conventional arms.

The sides ultimately want to reach a full agreement by June 30.

But both the US and Iran face pressure to come up with the main contours of a deal by this month's end, with US President Barack Obama and Iran's Ayatollah Ali Khamenei having spoken against extending negotiations for the third time.

Iran says the programme's aims are for energy, medical and research purposes, but the US and its allies in the Middle East, such as Israel, believe Iran harbours nuclear weapons ambitions.

Meetings in Brussels

In Brussels, French President Francois Hollande; David Cameron, UK prime minister; and German Chancellor Angela Merkel discussed the state of negotiations on Friday with Federica Mogherini, the EU's top diplomat.

All three countries are negotiating with the US, as are Russia and China.

Marie Harf, State Department spokesperson, said Kerry would meet the foreign ministers of Britain, France and Germany in London on Saturday, before returning to Washington DC.

The US is determined to maintain unity among its partners. But France, which raised last-minute objections to an interim agreement reached with Iran in 2013, could threaten a deal again.

It is particularly opposed to providing Iran with quick relief from international sanctions and is trying to secure a longer timeframe for restrictions on Iran's nuclear activity.

Source: Agencies