(Bloomberg) — Iran and world powers began their most serious attempt yet to resurrect a troubled nuclear deal, with negotiators from the U.S. and Islamic Republic gathered at the same venue for the first time since Donald Trump sent the accord into freefall in 2018.The talks at Vienna’s Imperial and Grand hotels on Tuesday may extend through the end of the week if progress is made, according to two officials involved. Iranian and U.S. representatives aren’t expected to speak directly, reflecting the deep distrust they will have to overcome. Iran said formal negotiations began at around 2:30 p.m. local time. Oil rebounded as the chances of a breakthrough were seen by analysts as slim, reducing the odds that crude flows from Iran would pick up further in the near term.Tensions over Iran’s accelerating nuclear program have threatened to spiral into open conflict since the Trump administration broke away from the 2015 agreement and reimposed punishing sanctions on an economy heavily dependent on oil. They’ve also sparked attacks on shipping in the Persian Gulf, and fueled the war in Yemen, including escalating strikes by Iran-backed rebels on Saudi Arabian oil facilities.The two main protagonists are far apart — Iran’s insisting that Washington must first guarantee the full lifting of sanctions for it to scale back its enrichment activity, while the Biden White House has ruled out any “unilateral gestures.”Robert Malley, the U.S. special envoy for Iran, is leading discussions for his government, while Iran’s team is headed by Abbas Araghchi, deputy foreign minister.Three European nations helping to broker the talks — Germany, France and Britain — aren’t expecting a major positive development on Tuesday, partly because of Iran’s insistence on U.S. sanctions being removed in a single step, said a European official who declined to be named because of the sensitivity of the matter. Oil ShipmentsA second European official said it’s important that progress is made before the end of May, referring to a 90-day deadline that Iran and the International Atomic Energy Agency agreed to on Feb. 22 in order to preserve the continuity of inspections data and give talks a chance. The official, who also asked not to be identified, put the probability of concrete action toward restoring the nuclear agreement before June at greater than 50%.Iran holds presidential elections that month, which could push its politics further toward hardline conservatives emboldened by the standoff with the U.S.“Iran’s demand for verifying sanctions relief could become a serious obstacle,” said Ali Vaez, director of the Iran Project at the International Crisis Group. Unlike Tehran’s nuclear activity, which is monitored by the United Nations, there’s no established procedure for ensuring effective relief from the penalties, he said.Ahead of the talks, China’s Foreign Ministry insisted it was up to the U.S., as the signatory that initially broke with the agreement, to send a strong, unconditional signal it’s ready for a deal. China, a signatory to the 2015 deal, has increased its leverage over Tehran in the wake of the U.S. withdrawal, most recently signing a decades-long economic cooperation agreement.‘Window of Opportunity’Tehran breached some of the limits on its nuclear program more than a year after Trump abandoned the deal that was championed by his predecessor, Barack Obama. It has since repeatedly rebuffed direct talks with the Biden administration on reviving the pact.Iran’s Foreign Ministry spokesman, Saeed Khatibzadeh, that the “window of opportunity” to break the deadlock with the U.S. won’t close if the talks don’t deliver a breakthrough. But he warned it “won’t remain open forever.”“The parties are not going to Vienna to waltz, but they might end up dancing around the main issues,” said Vaez, the analyst.For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2021 Bloomberg L.P.