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"; he and his French counterpart, Laurent Fabius, left Vienna saying they would return on Saturday, Reuters reported. 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 program in return for withdrawing economic sanctions that have crippled the Iranian economy. An agreement would be the biggest step toward rapprochement between Iran and the West since the 1979 Islamic Revolution.