A senior Iranian official has raised doubts about whether an agreement to curb Iran's nuclear programme could be finalised during high-level talks in Geneva.
Sky's foreign affairs correspondent Tim Marshall said "a deal does not look imminent" after a day of intense diplomacy, which saw US Secretary of State John Kerry and Britain's Foreign Secretary William Hague join foreign ministers from Russia, France, China and Germany.
Iranian Deputy Foreign Minister Abbas Araghchi told Iran's Fars news agency that the bargaining was primarily over the wording of a draft six-month agreement that would offer Iran limited relief from economic sanctions in return for curbs on its nuclear programme.
Mr Araghchi, who said "98% progress" had been made, added that it was unclear whether the remaining few differences could be ironed out on the fourth day of what was supposed to have been two days of talks.
He said Iran cannot accept any agreement that does not recognise its right to enrich uranium, which it claims is needed to produce electricity and for scientific research.
US officials insisted Mr Kerry would stick by plans to leave Geneva on Sunday for talks in London, which raised the possibility that another session may have to be scheduled.
Marshall said: "It feels like they are not drifting but inching towards accepting that they are still at stalemate and that they are going to have to try to do it (talks) again."
Earlier, Mr Hague told reporters: "They remain very difficult negotiations; I think it's important to stress that. We're not here because things are necessarily finished, we're here because they're difficult and they remain difficult."
The meeting, the third since Hassan Rouhani was elected as president of Iran in June, is seen as the biggest hope in years to resolve the stand-off over Iran's nuclear programme.
Failure might mean Iran resuming the expansion of its atomic activities, while Washington and others could toughen already painful sanctions and the possibility of Israeli military action would draw nearer.
At the last gathering, foreign ministers including Mr Kerry flew to Geneva, only for three days of talks to fail.
The goal is to hammer out an agreement to freeze Iran's nuclear programme for six months while offering the Iranians limited relief from crippling economic sanctions.
If the interim deal holds, the parties would negotiate final-stage agreements to ensure Iran does not build nuclear weapons.
Only then would the most crippling sanctions on Iranian oil sales and financial transactions be rolled back.
The talks have been made more difficult because each side has deep misgivings about dealing with the other.
The Iranians, mindful of opposition to any restrictions among hard-liners back home, have insisted on retaining the right to produce nuclear fuel by enriching uranium, saying they need it to produce electricity and for scientific research.
Mr Araqchi said on Saturday: "In the past 10 years, Iran has resisted economic and political pressures and sanctions aimed at abandoning its enrichment activities.
"Therefore any agreement without recognising Iran's right to enrich, practically and verbally, will be unacceptable for Tehran."
They are also holding out for maximum relief from sanctions that could hurt their economy.
The US and its allies want to relax sanctions in small, incremental steps, but even those have not been received well by Israel or some members of Congress.