Gaps have not been bridged on some key issues in the nuclear talks between Iran and world powers due to the "inadequate and unworkable" positions Iran holds, a senior U.S. official said on Saturday here.

After one-week intensive negotiations in the capital city of Austria, Iran and the so-called P5+1 states(the five permanent members of the United Nations Security Council plus Germany) made some progress but on some key issues Iran and world powers remained far apart.

"We made some progress, but on some key issues Iran has yet to move from their prospective unworkable and inadequate positions," the senior U.S. official told reporters in Vienna.

Under the interim deal agreed in Geneva last November, Iran agreed to suspend some sensitive nuclear activities in exchange of limited sanction relief in duration of six moths to buy time for the diplomatic effort to resolve the issue.

The six world powers are now working with Iran in Vienna to find a comprehensive solution to resolve the decade-long standoff between Iran and the West over Tehran's nuclear program by the deadline July 20.

The West wants Iran to significantly scale back its nuclear program to address its concern of proliferation risk, while Iran insists its nuclear right is inalienable.

The U.S. official said Iran's demand of uranium enrichment capacity in the future which was announced by Tehran's leader is far beyond the powers' limit, indicating the position between Iran and powers are still far apart on the uranium enrichment, the core issue of the talks.

Iran's supreme leader Ayatollah Ali Khamenei said on Tuesday Iran needs 190,000 separative work units (SWUs) for uranium enrichment, around many times higher than the West currently wants to allow under a comprehensive agreement.

French Foreign Minister Laurent Fabius said last month that Iran could retain "several hundred centrifuges" but he revealed that the Iranians were asking for "hundreds of thousands."

Iran currently has installed 19,000 centrifuges, of which 10,190 are operating.

Western states fear Tehran could produce nuclear fuel for nuclear weapon by uranium enrichment which Iran said is only for nuclear power plant.

The U.S. official also noted the duration of the comprehensive deal could be double digit, indicating the deal could be at least over 10 years.

U.S. Secretary of State John Kerry is to meet with other foreign ministers from P5+1 group on Sunday in Vienna, discussing the nuclear ongoing the nuclear talks.

The official told reporters that Kerry, together with the other ministers, would assess the situation of the nuclear talks to see if there could be substantive progress in the nuclear negotiations.

After one-week talks, some progress might be made in the future of Arak nuclear reactor, which could yield plutonium, another type of nuclear fuel for assembling a nuclear warhead, and also the Fordow nuclear enrichment plant.

Kelsey Davenport, the Nonproliferation Analyst from Arms Control Association told Xinhua that the compromise might be that Iran modifies the heavy water reactor to make it less powerful and to run on different fuel, reducing the plutonium output to make it less of threat for providing the fissile material for plutonium based nuclear weapons.

On the Fordow site, Davenport said Iran and powers might now accept it as a research and development facility for Iranian centrifuges. This would allow Iran to keep the facility open, but address the P5+1 concern about Fordow being used to produce uranium for weapons purposes. Endi