VIENNA : The United States and Iran traded barbs on Wednesday as negotiators arrived in Vienna for a marathon final round of nuclear talks aimed at securing a historic deal by a July 20 deadline.The five permanent members of the UN Security Council plus Germany want Iran to reduce its nuclear programme in order to ease long-held fears that Tehran might develop atomic weapons.Iran, subject to damaging UN and Western sanctions, insists its nuclear activities are purely peaceful.A sixth and final round of talks starts officially on Thursday and could potentially last until July 20 when an interim deal struck in November expires, although in theory this can be extended for six months.US Secretary of State John Kerry, writing in the Washington Post, said that the negotiations constituted “a choice for Iran’s leaders”.“They can agree to the steps necessary to assure the world that their country’s nuclear program will be exclusively peaceful and not be used to build a weapon”, Kerry wrote.Or “they can squander a historic opportunity to end Iran’s economic and diplomatic isolation and improve the lives of their people.”The P5+1 powers have proposed a “series of reasonable, verifiable and easily achievable measures that would ensure Iran cannot obtain a nuclear weapon and that its program is limited to peaceful purpose,” he said.“What will Iran choose? Despite many months of discussion, we don’t know yet”.But Iranian Foreign Minister Mohammad Javad Zarif, writing in French daily Le Monde, said that some among the P5+1 were suffering from “illusions” about Iran’s nuclear programme.He said that contrary to fears in the West, Iran - even if it wanted to - is “several years, not a few months” away from being able to build an atomic bomb.Iran “will not abandon or give up its technological or scientific advances. In addition it would not be prudent ... to expect us to do otherwise,” Zarif said in remarks printed in French.“I appeal for these illusions not to derail a process that could put an end to a pointless crisis,” he said.Iranian nuclear negotiator Majid Takhte Ravanchi went further, telling the ISNA news agency that Iran has set out clear “red lines” in the talks.“The other side knows that these red lines cannot be crossed. If we reach a deal it will be one respecting these red lines. If not there will be no accord,” he said.“The outcome depends on the other side. If they have a maximalist position, or if they want to address issues that have nothing to do with the talks, if they are far from reality or if they have illusions, there will be no deal,” he said.“We will not accept definitive restrictions” on our nuclear programme, he said.But Kelsey Davenport, Arms Control Association analyst, was upbeat about prospects for a deal, saying there is “considerable political will” and that an accord is in the interests of both sides.“There is a lot of time left for diplomacy and a good comprehensive nuclear agreement is within reach, despite significant gaps between the two sides on core issues,” she told AFP.