Serious differences persist between the United States and Iran over how they might resume compliance with the 2015 Iran nuclear deal despite making some progress in their latest indirect talks in Vienna, a senior U.S. official said on Wednesday. The talks were likely to require several rounds, their outcome remained uncertain and they were not near conclusion, the senior U.S. State Department official told reporters in a conference call. The main differences are over what sanctions the United States will need to remove and what steps Iran will need to take to resume its obligations to curb its nuclear program, said the official, who spoke on condition of anonymity. "There still are disagreements and, in some cases, pretty important ones," he said. "We are not near the conclusion of these negotiations. The outcome is still uncertain. We have made some progress." Earlier, the European parties to the Iran nuclear deal said they had seen progress in the first two rounds of indirect U.S.-Iran talks but there were still major hurdles. Iran and world powers - Britain, China, France, Germany and Russia - have been meeting in Vienna to hammer out steps that would be needed if the agreement, which was abandoned by former U.S. President Donald Trump in 2018, is to be revived. The talks will resume next week. A U.S. delegation is in a separate location in Vienna, enabling representatives of the five powers to shuttle between both sides because Iran has rejected direct talks. The central issues are which U.S. sanctions imposed by Trump since withdrawing would need to be removed and what steps Iran would have to take to rein it its subsequent breaches of the pact"s curbs on its uranium enrichment capacity. In the latest example of such breaches, Iran has installed extra advanced centrifuges at its underground uranium enrichment plant at Natanz that was hit by a blast last week, a report by the U.N. atomic watchdog showed. read more The Wall Street Journal cited two people familiar with the matter as saying Washington was open to easing terrorism sanctions against Iran"s central bank, its national oil and tanker companies and key sectors including steel and aluminium. Analysts have said it is inevitable the United States would have to ease some of its terrorism-related sanctions if there is to be a revival of the deal because without this, Iran would be unable to resume exporting oil. The newspaper cited one senior European official as saying that Washington has also signalled potential sanctions relief for sectors including textiles, autos, shipping and insurance. The U.S. official declined comment on the report, but said the U.S. delegation has given Iran examples of sanctions Washington believed it must remove to resume compliance, those it did not need to and would not remove, and a third category of "difficult cases" where the answer was uncertain. European officials said they hope to have something concrete in hand by mid-May, before a monitoring agreement between Iran and the U.N. nuclear watchdog expires later that month and ahead of the June 18 Iranian elections. The U.S. official said he would not rule out the possibility if enough progress were made, but stressed the United States would not rush to meet a deadline.