As what was supposed to be the final round of talks between the P5+1 group of world powers and Iran over a comprehensive agreement on the Iranian nuclear programme was heading to the end of its second, unscheduled week, it was unclear whether a deal could be reached, or whether a deadline to achieve one still existed.

The talks, in Vienna, had been extended until this Tuesday as negotiators continued to grapple with key issues.

As the deadline loomed, European Union foreign policy chief Federica Mogherini told the press that it would take "a few more hours or a couple of days".

On Tuesday afternoon, a United States State Department spokeswoman said the interim agreement, signed in November 2013, and intended originally to last for six months, was once again being extended - this time by three days until Friday night.

Only the indefatigable US Secretary of State John Kerry remained in Vienna after Tuesday, along with his Iranian counterpart, Foreign Minister Mohammed Jawad Zarif, as "expert" teams tried to thrash out the contentious clauses.

No deal had been agreed by Wednesday night, when the US Senate's foreign relations committee was scheduled to have started debating the agreement.

While some diplomats, particularly the Iranians, have insisted that they are on the brink of achieving an agreement, others emphasised that there are still significant issues to resolve.

French Foreign Minister Laurent Fabius said the sides had still not agreed on the procedure for resuming sanctions if Iran was found in breach of the deal, which fields of nuclear research would be frozen, and how the Iranians would come clean over the possible military dimensions of their nuclear activities.