By Louis Charbonneau and Fredrik Dahl
VIENNA (Reuters) - Iran will never slow down its nuclear research programme, the Islamic Republic's supreme leader said on Wednesday as negotiators from Tehran and six world powers struggled to narrow "significant gaps" in talks on a long-term accord.
Ayatollah Ali Khamenei said Tehran's negotiating team should not yield to issues "forced upon them".
"These negotiations should continue," he told nuclear scientists in Tehran, the official IRNA news agency reported.
"But all should know that negotiations will not stop or slow down any of Iran's activities in nuclear research and development."
The negotiators from Iran and the so-called P5-plus-one - the United States, Russia, China, Britain, France and Germany - plan after their two days of talks in Vienna to start drafting the agreement to meet a self-imposed July 20 deadline.
"The Iranians clearly have a sense of urgency to get a deal done, as does the P5+1 (the six powers)," a Western diplomat close to the talks told Reuters.
"We know that there are still some significant gaps that remain and know this process will not be easy. But we're all committed to getting it (the draft) done by July 20," he said, in an assessment echoed by other Western diplomats.
The toughest areas to be tackled are Iran's future uranium enrichment capacity, nuclear facilities that Western powers believe have little or no civilian value, and future nuclear research work, as well as a schedule of steps to remove the international sanctions that have crippled Iran's economy.
U.S. and European officials have said they will insist on limits to Iran's efforts to develop more efficient uranium enrichment technology, which would enable Tehran to produce sensitive nuclear material at a faster pace.
Background tensions over Russia's involvement in Ukraine and Western threats of further sanctions against Moscow and over the U.S. denial of a visa for Iran's proposed new U.N. envoy in New York have so far not harmed the nuclear talks, diplomats say.
A senior Iranian official said Tehran was seeking to protect its "red lines" in what he said were "difficult" negotiations.
"Iran wants a deal in which its rights have been considered," the official said. "The talks have entered a very difficult stage. Making progress is difficult."
The third negotiating session in Vienna this year will conclude on Wednesday with a meeting of senior foreign ministry officials from the six powers chaired by European Union foreign policy chief Catherine Ashton, who coordinates the talks for the group, and Iranian Foreign Minister Mohammad Javad Zarif.
The six powers' goal is to extend the "breakout" period Iran would need to develop an atomic weapon as much as possible. U.S. Secretary of State John Kerry said on Tuesday the current Western view of that period is two months.
Khamenei has repeatedly said that the oil-producing OPEC member's "red lines" are that it will never give up enrichment or shut any nuclear facility.
Among the toughest issues are Iran's centrifuge research and development programme, the size of its uranium stockpiles, the future of the Arak research reactor that could eventually yield significant quantities of bomb-grade plutonium, and the future of the previously hidden Fordow underground enrichment plant.
The stakes are high. Western powers, along with Russia and China, want to avert an escalation of tensions in the Middle East in the form of a new war or a regional nuclear arms race. Israel, believed to be only nuclear-armed nation in the Middle East, has threatened to take military action against Iran if it is not satisfied that the nuclear programme is curbed.
Iran, which denies accusations it is seeking a nuclear weapons capability, wants an end to sanctions that have drastically reduced its oil income and virtually barred it from the international financial system and to regain what it sees as its rightful place as a leading regional power.
The current Vienna talks are building on a preliminary deal that Iran and the six powers reached in Geneva last November. That agreement provided Iran with limited sanctions relief in exchange for a six-month suspension of some nuclear activities that began on January 20.
The head of the U.N. nuclear watchdog in Vienna, Yukiya Amano, told Reuters in Oslo that Iran is cooperating with his inspectors, who are seeking answers about detonators that could be used to help set off an atomic explosive device, part of a wider investigation into Tehran's activities.
Western negotiators say that clarifying Iran's past nuclear activities is essential if they are to be able to accurately predict what Iran's future breakout capability will be.
(Additional reporting by Parisa Hafezi, Fredrik Dahl and Justyna Pawlak in Vienna and Mehrdad Balali in Dubai; Editing by Gareth Jones and Angus MacSwan)