Global powers sought Thursday to ramp up pressure on Iran as they struggled to strike a long-sought nuclear accord, with one Iranian official admitting "God only knows" if a deal was close.

With speculation rife about the fate of the tough negotiations, US Secretary of State John Kerry was to give an update on the situation to reporters at 1700 GMT, a US official said.

All bets remained off as negotiators held back-to-back meetings before Friday's effective deadline, hoping to end a 13-year standoff with Iran over its suspect nuclear programme.

If the US Congress does not receive the text by early Friday morning Vienna time -- midnight in Washington -- it will likely delay and complicate its implementation.

But an Iranian official insisted to AFP: "For us, no date is sacred if it means sacrificing a good accord."

Two deadlines have already been missed during this round of negotiations alone which have now stretched into their 13th day.

Diplomats from various delegations ruled out a deal by the end of Thursday, but when asked whether an accord would be possible Friday or Saturday, one Iranian official told AFP: "God only knows."

After almost two years of continuous negotiations, the so-called P5+1 group -- Britain, China, France, Germany, Russia and the United States -- wants to wrap up a deal to deny Iran the capability to make a nuclear bomb, in return for lifting biting international sanctions.

With the talks seemingly at deadlock, French Foreign Minister Laurent Fabius rejoined Kerry as well as his German and British counterparts early Thursday, alongside Chinese and Russian officials, to plot a strategy forward.

Kerry then met briefly with Iranian Foreign Minister Mohammad Javad Zarif, and was due to hold another P5+1 meeting without Iran later in the day, before addressing the press.

- Spanner in the works -

All sides say huge progress has been made in the past days of negotiations -- the final stage of marathon talks first launched in September 2013.

Most of the final text is already written, along with most of five technical annexes.

Some of the thorniest issues such as a time frame for lifting sanctions, a UN probe into allegations that Iran in the past sought to develop nuclear weapons, also appear close to resolution.

But Iran's demand that a UN arms embargo be lifted has thrown a spanner in the works. Western nations balk at the idea, with Tehran still accused of fomenting violence in the Middle East.

Russian Foreign Minister Sergei Lavrov, who was in the Russian city of Ufa at a summit with emerging economies, threw Moscow's weight behind Tehran.

"We are in favour of lifting the embargo as soon as possible and will support a decision made by Iran's negotiators," Lavrov told reporters.

The UN Security Council arms embargo had been imposed to force Iran to negotiate, a goal that had "long been reached," he stressed, adding that resuming arms deliveries would help Iran combat terrorism and radicals from the Islamic State group.

There were no "insurmountable problems" left to tackle at the talks, unless somebody tried to deliberately stall the negotiating process, Lavrov insisted.

- Deal not good enough yet -

Kerry late Wednesday held a long video conference with US President Barack Obama to review progress so far, after the American leader reportedly told senators earlier that the chances of an accord were "less than 50-50."

"We don't have an agreement that's good enough yet," the US delegation's spokeswoman Marie Harf told CNN.

"It doesn't have all the assurances we need, it doesn't meet our bottom line, so we are taking more time."

Iran has long denied seeking nuclear arms. Building on a framework agreement from April, the deal would see Iran dismantle and mothball large parts of its nuclear infrastructure.

In return, a web of painful sanctions on Iran would be progressively lifted once the UN atomic watchdog verified Tehran had fulfilled its promises.

Kerry quipped Thursday at the start of a new meeting that they were in "Groundhog Day".