Major powers turn the screws for Iran nuclear deal.

Vienna

Foreign ministers from major powers sought Thursday to turn the screws on a hard-bargaining Iran to finalise a historic nuclear deal on the eve of a deadline to present it to US lawmakers.

If the US Congress does not receive the text by early Friday morning Vienna time midnight in Washington it makes the approval process longer and potentially more problematic.

But despite this pressure, and almost two weeks of talks in the Austrian capital, it is unclear whether an accord aimed at ending a 13-year standoff can be sewn up at all.

US President Barack Obama held a video conference Wednesday with the US negotiators and "reviewed the progress of negotiations to date, and provided guidance" to the team, the White House said.

But Obama had reportedly told senators on Tuesday that the chances of an accord were "less than 50-50."

Building on a framework agreement from April, the deal would see Iran dismantle large parts of its nuclear infrastructure in order to put a nuclear bomb out of reach.

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