"We are continuing to negotiate for the next couple of days," EU foreign policy chief Federica Mogherini told Reuters.
The deadline had previously been extended from 30 June until 7 July, but it was clear that more time was needed, US delegation spokeswoman Marie Harf told Reuters.
"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," Harf said, according to Reuters.
"No deadline is sacrosanct for us," Iranian negotiator Abbas Araqchi told Reuters. "We are ready to stay in Vienna and continue talks as long as it is necessary."
However, a source told Reuters that this is the final extension. "We've come to the end," said the source, on condition of anonymity. "It is hard to see how or why we would go beyond this. Either it happens in the next 48 hours, or not."
This is the fourth time sanctions relief has been extended. Sanction relief plans were set out in the 2013 Joint Plan of Action (JPOA) and ease certain sanctions in return for voluntary measures from Iran.
The JPOA eases US and EU sanctions on petrochemical, gold and precious metal exports from Iran, plus US sanctions on the auto and aviation industries. It prohibits further nuclear-related sanctions, creates a financial channel for humanitarian trade and increased the threshold on monetary transactions involving Iran.
Oil and gas expert George Booth of Pinsent Masons, the law firm behind Out-Law.com has previously warned companies against jumping in too quickly when trade restrictions are finally lifted.
Today, Booth said: "Clients are already undertaking market analysis and vetting possible partners in expectation of relaxation of the sanctions – and understandably so. One of the keys to successful investments into Iran will be the adoption of a flexible investment model, which is able to meet the commercial aspirations of both the Iranian party and the overseas investor however the legal, political and regulatory landscape develops following relaxation – as it undoubtedly will," he said.
