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Updated: Jul 08, 2015 13:06 IST

Ongoing talks towards securing a historic nuclear deal between Iran and major global powers went into overtime on Wednesday after stumbling over, as one Western diplomat said, "very, very, very tough" remaining issues.

Iran and six world powers have effectively given themselves until Friday to reach a deal by extending the terms of a 2013 interim accord under which Tehran has frozen parts of its nuclear programme in return for minor sanctions relief.

This is the latest in a string of extensions aimed at ending almost two years of talks to resolve a 13-year standoff with the Islamic republic after foreign ministers failed to bridge the last few gaps in Vienna last week.

"Removing the remaining brackets (in the text of the agreement), this seems to be very, very, very tough," the senior diplomat said Tuesday as an 11th day of talks stretched late into the night.

But the envoy insisted the negotiations, originally due to end on June 30, are "not an open-ended process. We've given ourselves a couple more days because we think it can be done."

This was rammed home by a second diplomat, who said the new target date is the "final" one.

"It's difficult to see why and how we could go on any longer. Either this works in the next 48 hours or it doesn't," the second diplomat said on condition of anonymity.

"We have never been closer, than we've ever been on this agreement, and we are still not where we need to be to finalise a deal," a senior US administration official said.

US Secretary of State John Kerry has remained in Vienna with his Iranian counterpart Mohammad Javad Zarif.

Their Russian, Chinese, French and British counterparts -- which along with Germany make up the P5+1 group in talks with Iran -- had already left. The latter two said they would return to the Austrian capital on Wednesday evening.

But for many observers July 9 had always been the real deadline. If Kerry fails to hand over a deal by the end of Thursday, US lawmakers will get 60 days instead of 30 to review it, potentially making it even more complicated to implement.

The US team now has its back against the wall trying to nail down the final details by then.

Arms ban to remain

The mooted deal would curb Iran's nuclear programme for a decade or more in order to make any push to make nuclear weapons virtually impossible -- an intention Iran has always denied -- in return for progressively lifting sanctions.

Despite progress on a series of complicated annexes, negotiations have stalled on how to ease sanctions against Iran, probing allegations that in the past Tehran did try to develop nuclear arms and ensuring Iran can continue to have a modest, peaceful nuclear programme.

Russian Foreign Minister Sergei Lavrov confirmed Tuesday a "major issue" was disagreement over the issue of lifting of a UN arms embargo, which bans sales of convention weapons such as tanks and missiles to Tehran.

While Iran had managed to develop its own arms industry, global powers "must change their approach on sanctions if they want a deal," the country's lead negotiator Abbas Araghchi said, calling for the UN ban to be changed.

But US officials insisted there would be "ongoing restrictions on arms just like there will be ongoing restrictions regarding missiles" in any nuclear deal, which is to be endorsed by a resolution in the UN Security Council.

Negotiators are already drawing up a draft resolution, which would also address the nuclear-related bans on arms trade and ballistic missiles, the senior administration official said.

While Iran has a right to conventional missiles "what we are concerned about is missile technology that becomes a delivery system for a nuclear weapon".