Six world powers and Iran will need "a lot of intensive work" to bridge differences during talks over Tehran's nuclear program, the European Union's foreign policy chief Catherine Ashton said on Wednesday after their latest meeting.
Ashton added that a new round of talks would start on May 13, also in the Austrian capital.
This comes as the Islamic Republic's Supreme Leader was reported as saying that Iran should continue talks with world powers to end a long-running nuclear dispute, but without ceding any of the gains made by its nuclear program.
Iran's negotiators should not yield to issues "forced upon them", Ayatollah Ali Khamenei said in remarks to nuclear scientists in Tehran, the official IRNA news agency reported.
He added that Washington knew well that the Islamic Republic was not seeking a nuclear weapon.
Meanwhile, Iranian Foreign Minister Mohammad Zarif said that the Islamic Republic and major powers had reached agreement on up to 60 percent of the issues regarding the country's disputed nuclear activities.
"We have agreement over 50 to 60 percent of the (final) draft ... but the remaining parts are very important and contain various issues," Zarif told reporters.
Zarif also said the Islamic Republic would not accept any pressure over its nuclear activities.
"Nothing can be imposed on Iran regarding its nuclear activities," Zarif told reporters after two days of talks in Vienna with the six major powers.
Meanwhile, a senior Chinese official at nuclear talks between six world powers and Iran, held in Vienna on April 8-9, said he was optimistic a comprehensive settlement to the decade-old dispute could be found by a 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.
"This is actually definitely helpful to the overall result of the dialogue," he said.
Negotiators from Iran and six world powers struggled on Wednesday to narrow "significant gaps" in talks aimed at clearing the way for a long-term accord on curbing Tehran's nuclear program.
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 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.
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.
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.
Iran's Supreme Leader Ayatollah Ali 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 program, 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.
Iran, which denies accusations it is seeking a nuclear weapons capability, wants an end to sanctions 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.
REUTERS