The European parties to the Iran nuclear deal have seen progress in the first two rounds of negotiations to revive the 2015 accord but said on Wednesday that there were still major hurdles to overcome. The talks, aimed at bringing Iran and the United States back into compliance with the agreement, will pick up again next week. Iran and world powers – Britain, China, France, Germany and Russia – have been meeting in Vienna since early April to hammer out steps that must be taken, touching on U.S. sanctions and Tehran’s breaches of the deal’s limits on its uranium enrichment capacity. A U.S. delegation is in a separate location in Vienna, enabling the powers to shuttle between both sides. “We welcome the constructive discussions that have taken place in Vienna, and the positive participation by all sides so far,” diplomats from France, Britain and Germany, known as the E3, told reporters.