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 stand-off 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 as an 11th day of talks stretched late into the night in Vienna.
But the envoy insisted the negotiations 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 remained in Vienna with his Iranian counterpart Mohammad Javad Zarif.
Their Russian, Chinese, French and British counterparts had already left. The latter two said they would return to the Austrian capital on Wednesday evening.
"If very tough political decisions, hard choices, can get made soon, I do believe we can get to an agreement ... it is possible," the US official said.
For many observers July 9th had always been the real deadline, and the US team now has its back against the wall trying to nail down the final details by then.
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, which risks further complicating its implementation.
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 -- it denies any such aim -- virtually impossible.
In return painful sanctions on Iran would be progressively lifted.
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 sought to develop nuclear arms, and ensuring Iran can continue to have a modest, peaceful nuclear programme.
Russian Foreign Minister Sergei Lavrov confirmed there was also disagreement over the issue of lifting of a UN conventional arms embargo which bans sales of conventional weapons such as tanks and missiles to Tehran.
"I can assure you that there remains one major problem that's related to sanctions: this is the problem of an arms embargo," Lavrov told Interfax from Vienna.
The arms embargo was not overly important as Iran had developed its own industry, but global powers "must change their approach on sanctions if they want a deal," Iran's lead negotiator Abbas Araghchi said late Tuesday, saying the UN ban had to be changed.
"Western nations must be prepared to give up sanctions," Araghchi said.
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."