Iran nuclear deal deadline may 'slip': US

High-stakes talks to nail down a historic deal with Iran to curtail its nuclear program may "slip" past a June 30 deadline, a top US official has admitted ahead of crunch weekend negotiations in Vienna.

"We may not make June 30, but we will be close," the senior official told reporters on Thursday as top US diplomat John Kerry prepared for Friday's talks, which could be the last between Iran and global powers on the deal.

The official said the target date to finalise the historic deal - the main outlines were agreed in April in Lausanne, Switzerland - would only "slip" by a few days.

"The intent of everybody here - the P5+1, the European Union, Iran - is to stay until we get this done, or find out we can't. And our intent is to get it done," the official told reporters, asking not to be named.

Others, including Iranian Foreign Minister Mohammad Javad Zarif, who is due in Vienna on Saturday, have said the deadline may be missed by a few days but until now Washington had insisted it still aimed to get a deal on schedule.

Iran, which has engaged in something of a rapprochement with the West since the election of President Hassan Rouhani in 2013, denies wanting nuclear weapons, saying its nuclear program is exclusively peaceful.

Under the Lausanne framework, which was also agreed several days late, Iran will downsize its nuclear activities in order to make any attempt to make a nuclear weapon all but impossible.

This includes Iran cutting the number of centrifuges enriching uranium, which is used in nuclear power but also for a bomb when highly purified, as well as slashing its uranium stockpiles and changing the design of a new reactor.

Tighter inspections by the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, would give ample notice of any "breakout" to make a bomb.

In return, UN and Western sanctions that have caused Iran major economic pain would be progressively lifted, although the six powers insist they can be easily "snapped back" in place if Tehran violates the accord.

French Foreign Minister Laurent Fabius and EU foreign policy chief Federica Mogherini are also expected to arrive this weekend.

The mooted final accord between Iran and the "P5+1" - the United States, China, Russia, Britain, France and Germany - will set out an exact timetable of sanctions relief and reciprocal steps by Iran, as well as a mechanism for handling possible violations by either side.

Tricky issues include how UN sanctions might be re-applied, the reduction of Iran's uranium stockpile and its future research and development on newer, faster types of centrifuges.