Negotiators have made significant progress in the last week on reviving the 2015 Iran nuclear deal but very tough issues remain, a senior US State Department official said on Friday. The US official said he hoped Iran’s lead negotiator would return in the coming days to Vienna, where the talks are taking place, “with a positive view” but that even if he did, there were still difficult issues on the table. “We hope that when Iran comes back, it comes back in with a pre-disposition to try to resolve this quickly,” the official told reporters on condition that he not be named. “But there were still disagreements for which there is not a solution thats on the table,” Reuters quoted him as saying. He declined to name the sticking points and would not be drawn on whether Washington had persuaded Tehran to agree to follow-on negotiations on its nuclear program, its development of ballistic missiles or support for regional proxies. The broad aim of the talks is to return to the original 2015 bargain of lifting sanctions against Iran, including those that have slashed its oil sales, in exchange for restrictions on its nuclear activities that extend the time Tehran would need to make enough enriched uranium for an atomic bomb if it chose to. The US official said a deal, if one can be reached, would in many ways track the terms of the original accord on Iran’s levels of uranium enrichment, the stockpile of enriched uranium it may hold, and the numbers of centrifuges it may operate. However, he left open the possibility of some modifications to account the additional sanctions that then-President Donald Trump imposed on Iran after pulling the United States out of the deal in 2018 and the nuclear advances that Iran has since made. The official also said there has not been any deal reached in separate negotiations about the release of four US citizens whom the United States believes have been wrongfully detained by Iran. Last week, sources close to the negotiations said a prisoner swap between Iran and the United States is expected soon. “Now I believe some of them will be released, maybe five or six of them. But those talks about prisoners are not linked to the nuclear agreement, rather associated with it. This is a humanitarian measure by Iran,” Reuters quoted a senior Iranian official as saying on Tuesday. State Department spokesperson Ned Price said on Friday that the US will continue to engage with Russia over efforts to prevent Iran from developing nuclear weapons, even though Moscow’s invasion of Ukraine had made it a “pariah on the world stage.” Price said US officials would now only engage with Russia counterparts on issues of “fundamental to our national security interest,” including Vienna talks. “The fact that Russia has now invaded Ukraine should not give Iran the green light to develop a nuclear weapon,” Price stressed.