Tense Iran nuclear talks to miss deadline

Iran and major powers have admitted during tense talks that their fast-looming deadline to nail down a historic nuclear deal will be missed as they struggle to overcome major differences.

Officials in Vienna said on Sunday however that Tuesday's target date would only be missed by a few days, with Iran saying there is "no desire or discussion yet" on a longer extension.

Iranian Foreign Minister Mohammad Javad Zarif meanwhile was set to return to Tehran for consultations, officials said, although the US said this was not a matter of concern.

British Foreign Secretary Philip Hammond said as he joined the talks with US Secretary of State John Kerry and other foreign ministers that the six powers were prepared to walk away if needed.

"We still have very big challenges if we are going to be able to get this deal done," Hammond told reporters.

"No deal is better than a bad deal. There are red lines that we cannot cross and some very difficult decisions and tough choices are going to have to be made by all of us," Hammond said.

Earlier on Sunday, EU foreign policy head Federica Mogherini said "political will" was still needed to get a deal after almost two years of intense diplomatic efforts to resolve the 13-year-old standoff.

"It is going to be tough, it has always been tough but not impossible," Mogherini told reporters.

Iran and the P5+1 group - Britain, China, France, Germany, Russia and the United States - are seeking to flesh out the final details of a accord that builds on a framework deal reached in Lausanne in April.

Zarif's deputy Abbas Araghchi suggested parts of that framework no longer applied because other countries had changed their positions.

A deal must stand up to intense scrutiny by hardliners in Iran and the United States, as well as Iran's regional rivals Israel, widely assumed to have nuclear weapons itself, and Saudi Arabia.

According to the Lausanne framework, Iran will slash the number of its uranium enrichment centrifuges, which can make nuclear fuel but also the core of a bomb, shrink its uranium stockpile and change the design of the Arak reactor.

In return it is seeking a lifting of a complicated web of EU, US and UN sanctions which have choked its economy and limited access to world oil markets.

But tough remaining issues include the timing and pace of this sanctions relief and UN access to Iranian military bases, something which Iran's supreme leader, Ayatollah Ali Khamenei, on Tuesday again ruled out.

German Foreign Minister Frank-Walter Steinmeier said ahead of his arrival in Vienna that "if there is no clarity on this (inspections of military sites), there will be no deal."