Negotiations between Iran and the world powers over Tehran's nuclear programme failed to reach their conclusion by the 7 July deadline. The diplomats have given themselves at least until 10 July to reach a deal.
"We are continuing to negotiate for the next couple of days," EU foreign policy chief Federica Mogherini said outside the hotel where the marathon talks between Iran and the P5+1 Group - Britain, China, France, Germany, Russia and the United States - took place.
US spokeswoman Marie Harf said: "We're frankly more concerned about the quality of the deal than we are about the clock, though we also know that difficult decisions won't get any easier with time."
Senior Iranian negotiator Abbas Araqchi hinted that there would be no deadline. "No deadline is sacrosanct for us," he told reporters. "We are ready to stay in Vienna and continue talks as long as it is necessary."
There's a time limit - but no one knows how long
However, a source from one of the powers told Reuters that there would be a time limit.
"We've come to the end," said the source, on condition of anonymity. "We have just made one, final extension. It is hard to see how or why we would go beyond this. Either it happens in the next 48 hours, or not."
The talks have hit a stumbling block over the lifting of a UN arms embargo. While Iran wants it lifted immediately irrespective of the outcome of the talks, the US says no. Sources said there was a full and frank exchange of views.
"There was no slamming of doors but it was a very heated exchange of views," a senior Western diplomat told reporters.
The Obama administration has its own political deadline
The 7 July deadline had been set as the date by which the Obama administration would have to submit a deal to Congress to avoid its review period doubling from 30 to 60 days.
Senior Democrat Representative Steny Hoyer told reporters: "This is a very serious agreement - I think [an extended deadline is] not bad.
"My concern has been that there will be a rope-a-dope sort of performance by Iran and they'll just string out these negotiations," he explained.
At the same time, Hoyer agreed with Senate Foreign Relations Chairman Bob Corker, a Tennessee Republican, who has said recently that negotiators should continue discussions if needed past the deadline because "time limits ought not to be the issue here".