VIENNA (AFP) - Tough talks on a deal ending a 13-year standoff over Iran's nuclear programme headed into a third successive weekend, as top US diplomat John Kerry said Friday some issues had been resolved.

After a bleak couple of days with both sides trading accusations over the stalemate in the dragging negotiations, Kerry praised the "very constructive" atmosphere on Friday.

Iran and the so-called the P5+1 group -Britain, China, France, Germany, Russia and the United States - are seeking to curb Teheran's ability to develop nuclear weapons in exchange for relief from painful sanctions.

Despite almost two years of negotiations, this round of talks touted as the last push, have moved at a snail's pace, with tempers fraying and Teheran and Washington publicly bickering over who is to blame.

To keep the negotiations going, the European Union and the United States have now prolonged the terms of a 2013 interim deal until Monday, after a series of missed deadlines.

"I think we have resolved some of the things that were outstanding and we've made some progress," Kerry said, as he met with his team of experts at the Palais Coburg hosting the talks in Vienna.

French Foreign Minister Laurent Fabius and his British counterpart Philip Hammond were set to return to the Austrian capital Saturday, having joined the other P5+1 ministers earlier Friday.

"We're making progress, it's painfully slow... there are still some issues that have to be resolved," said Hammond.

But he voiced hope that over the next hours experts working behind the scenes "will clear some more of the text and then we can re-group tomorrow (Saturday) to see if we can get over the last hurdles."

Almost two years after Hassan Rouhani's election as Iranian president, a ground-breaking deal could see the Islamic Republic's relations with the West thaw after 35 years in the deep freeze.

But there are powerful forces opposed, not least US President Barack Obama's Republican opponents and Iran's regional rivals Saudi Arabia and Israel, widely assumed to have nuclear weapons itself.

"If the West gives up its excessive demands, we will certainly have a good deal in the nuclear negotiations with the P5+1," Iran's First Vice President Es-Hagh Jahanguiri told Iranian media.

He was speaking as tens of thousands in Teheran marked the annual "Quds Day" in support of the Palestinians, shouting "Death to America" and "Death to Israel".

Iranian Foreign Minister Mohammad Javad Zarif late Thursday speaking on Al-Alam television had accused the West of "changes in the position and excessive demands."

BOGGED DOWN

Kerry, meanwhile, warned the US would not be rushed into a deal, but insisted he would not stay in Vienna forever.

"If the tough decisions don't get made, we are absolutely prepared to call an end to this process," Kerry told reporters.

Russian President Vladimir Putin said Friday he believed "a compromise should be found" after hosting a summit of the Shanghai Cooperation Organisation in the Urals city of Ufa.

"In my opinion, it will be found soon," he said.

Armies of technical and legal experts have made great strides working out the legal and technical nuts and bolts of what will be a fiendishly complex final agreement.

But they have got bogged down working out how exactly the deal will be implemented, including a timetable for dovetailing the lifting of sanctions with dismantling Iran's nuclear programme.

Another major problem is Iran's push to have a UN arms embargo lifted, something which appears to have driven a wedge between the Western powers on one side and China and Russia on the other.

"If the remaining gaps are not bridged by the end of this weekend, a break might prove inevitable," International Crisis Group analyst Ali Vaez told AFP.

"But leaving Vienna empty handed is more likely to stiffen the parties' positions than to soften them," he said.