US President Barack Obama has been pushing to secure a historic nuclear deal with Iran Getty

US President Barack Obama believes the chances of reaching a nuclear deal with Iran is "less than 50-50" as negotiations between the parties missed yet another self-imposed deadline.

The president made the remark in a closed-doors meeting on 7 July, according to Democratic Senator Dick Durbin.

"He said the chances, he thought, were less than 50-50 at this point and that he wouldn't agree to something he thought was weak or unenforceable," Durbin told Politico in an interview.

"But if he comes up with an agreement and it meets his standards, he wanted us to take an honest look at it and not prejudge," he added.

Obama's remarks come as the deadline for negotiations between Iran and the so-called P5+1 group of powers – the US, UK, France, Russia, China and Germany – were extended by three days to 10 July.

The talks in Vienna have hit a stumbling block over the lifting of a UN arms embargo. While Iran wants it lifted immediately, the US and its European allies are against the move.

Obama "covered every hot topic there was" when it came to Iran at the meeting on 7 July, according to Democratic Senator Chris Murphy.

"He wanted to make it clear to us that if it's a bad deal, there's a no deal," he said.

The extension of the deadline means any agreement submitted to the US Congress for approval would take a review period of 60 days, rather than the 30 days it would take if it is tabled by 9 July.

The US Congress has the right to reject any deal reached with Iran, although Obama would be able to use his presidential veto to overrule the Congress.

Some Republicans have opposed a deal with Iran, saying far too many concessions have been made in the negotiations.