NEW YORK – Gaps remain between the West and Iran over its nuclear work, American officials said on Wednesday at the conclusion of two days of negotiations in Vienna.But world powers are nevertheless ready to draft a comprehensive agreement on Iran’s expansive nuclear program.“Now we are set to start drafting,” a senior Obama administration official told reporters in Austria at the conclusion of the talks. Iran and the P5+1 powers – the United States, Britain, France, Germany, Russia and China – have agreed to give themselves until July 20 to negotiate a comprehensive solution to the impasse over the Iranian program, now spanning over 20,000 uranium-enriching centrifuges across several plants and with a heavy-water plutonium plant under construction, providing Iran with two tracks toward nuclear weapons capability.The US official shared cautious optimism with reporters after the talks, noting that significant differences remain on key issues, including Iran’s retention of the plutonium facility in Arak.“At this point, we don’t know if we’ll be successful in bridging those gaps,” the official said.Chinese official expressed optimism that a comprehensive settlement to the decade-old dispute could be found by the July 20 deadline.“Everyone is aiming at that,” the official, Wang Qun, told reporters after the two-day talks. He also said that Russia was playing a constructive role in the negotiations despite mounting tensions between Moscow and Western governments over Ukraine.Talks ended in Vienna as Tehran marked Nuclear Technology Day, a celebrated event on the Iranian government calendar.Iranian Supreme Leader Ayatollah Ali Khamenei spoke about Iran’s nuclear work to a small group of nuclear scientists and officials and took the opportunity to chide the United States.“Americans are well aware we are not after nuclear weapons, but they still raise the charges every now and then to keep up the anti-Iran hype,” Khamenei said, according to Iran’s state-run news agency. “The decision to negotiate doesn’t mean we will backtrack on the issue.”In Vienna, Iran’s Foreign Minister Mohammad Javad Zarif said the majority of differences with the West were already bridged.“We have agreement over 50 to 60 percent of the draft, but the remaining parts are very important and contain various issues,” he said.“Nothing can be imposed on Iran regarding its nuclear activities,” Zarif said. Reuters contributed to this report.