VIENNA - Iran and major powers gave themselves until Monday to reach a nuclear agreement, their third extension in two weeks, as Tehran accused the West of throwing up new stumbling blocks to a deal.

Both sides say there has been progress in two weeks of talks, but British Secretary Philip Hammond called it "painfully slow" and he and his French counterpart, Laurent Fabius, left Vienna saying they would return on Saturday. Having missed a Friday morning U.S. congressional deadline, U.S. and European Union officials said they were extending sanctions relief for Iran under an interim deal through Monday to provide more time for talks on a final deal.

Iran and six powers - Britain, China, France, Germany, Russia and the United States - are trying to end a more than 12-year dispute over Iran's atomic program by negotiating limits on its nuclear activities in exchange for sanctions relief. The sides remain divided over issues that include a U.N. arms embargo on Iran which Western powers want to keep in place, access for inspectors to military sites in Iran and answers from Tehran over past activity suspected of military aims.

Iran's foreign minister Mohammad Javad Zarif said a deal was unlikely to be reached on Friday and negotiators would probably spend the weekend in Vienna. He sought to blame the West for the impasse. "Now, they have excessive demands," he said of the major powers' negotiating position. Britain's Hammond said ministers would regroup on Saturday to see if they could overcome the remaining hurdles.

"We are making progress, it's painfully slow," he told reporters before leaving Vienna. Zarif has been holding intense meetings for two weeks with U.S. Secretary of State John Kerry to try to hammer out a deal limiting Iran's nuclear programme in return for withdrawing economic sanctions that have crippled the Iranian economy.

An agreement would be the biggest step towards rapprochement between Iran and the West since the 1979 Islamic Revolution. But the negotiations have become bogged down, with final deadlines extended three times in the past 10 days and diplomats speaking of a shouting match between Kerry and Zarif. The White House said on Friday the United States and its negotiating partners "have never been closer" to agreement with Iran but that the U.S. delegation would not wait indefinitely.

"The president has indicated to his negotiating team that they should remain in Vienna and they should continue to negotiate as long as the talks continue to be useful," White House spokesman Josh Earnest said at a news briefing. "And if it becomes clear that Iran is not interested in engaging in a constructive way to try to resolve the remaining sticking points, then the negotiators should come home."

The negotiators missed a Friday morning deadline set by the U.S. Congress for an expedited 30-day review of the deal. Any deal sent to Congress before Sept. 7 would now be subject to a 60 day review period, accounting for lawmakers' summer recess. US officials had previously expressed concern that the extended review would provide more time for any deal to unravel, but have played down that risk in the last few days as it became increasingly likely that the deadline would not be met.

"We still have a couple of very difficult issues, and we'll be sitting down to discuss those in the very near term - this evening and into tomorrow." The talks are now heading into their third weekend in Vienna as global powers and Tehran seek to bridge the final gaps blocking a long-sought deal.

They are seeking to end a 13-year standoff with Iran over its suspect nuclear programme with a deal which will block its pathways to developing a nuclear bomb in exchange for lifting biting global sanctions. The terms of a 2013 interim accord under which Iran has suspended much of its uranium enrichment in return for some sanctions relief were extended to Monday in a bid to overcome a deadlock. France's Laurent Fabius and British Foreign Secretary Philip Hammond are also due to return to the Austrian capital Saturday in the hopes of advancing the deal.