A Democratic minority in the US Senate staved off a united Republican effort to sink the Iran nuclear deal Thursday, handing President Barack Obama a welcome foreign policy victory.

As expected, Senators fell two votes shy of the 60 needed to advance a resolution disapproving of the international accord, meaning the legislation aimed at sabotaging the deal is essentially dead.

"The Senate has spoken with a clarion voice and declared that the historic agreement to prevent Iran from obtaining a nuclear weapon will stand," Senate Minority Leader Harry Reid said after Democrats cleared the way for the accord.

The result effectively assures that the deal, in which Iran has agreed to roll back its nuclear program in exchange for an easing of crippling economic sanctions, will go into force while sparing Obama the embarrassment of having to use his veto pen against a resolution of disapproval.

But Republicans, who unanimously oppose the accord, vowed to keep fighting, with House Speaker John Boehner insisting "this debate is far from over, and frankly it's just beginning."

Boehner said House Republicans will "use every tool at our disposal to stop, slow and delay this agreement from being fully implemented," including suing the president to prevent him from carrying out the Iran accord.

"That is an option that is very possible," Boehner told reporters Thursday.

Congress had passed legislation that gave it 60 days to review the July accord, and play the "spoiler" role if it deemed the agreement was not in the security interests of the United States.

That 60-day period expires on September 17.

"That will be good news, and it will mean that the international community can move forward with implementing the agreement," White House spokesman Josh Earnest said Thursday in anticipation of the Senate vote.

The deal struck between Iran and six world powers provides Tehran relief from crippling economic sanctions in exchange for limits on its nuclear program.

Republicans complain the deal does not do away with the program altogether, fails to provide for spot inspections of nuclear sites or force Iran to end support for militant groups like the Palestinian Islamist movement Hamas.

Cameron, Hollande and Merkel urge support for Iran deal
Washington (AFP) Sept 10, 2015 - The leaders of Britain, France and Germany jumped into the fierce political debate in Washington over the Iran nuclear deal Thursday, in an effort to convince US lawmakers not to derail the agreement.

As members of the P5+1 group of world powers, the European allies were party to the agreement that saw Iran agree to submit to international controls on its nuclear program in exchange for sanctions relief.

US President Barack Obama has hailed the deal as the best way to prevent Iran building a nuclear bomb that would upset the strategic balance in the Middle East, but has run into domestic opposition.

On Thursday, Prime Minister David Cameron, President Francois Hollande and Chancellor Angela Merkel published a joint op-ed in the Washington Post, aimed at convincing the US Congress to back the deal.

"We are confident that the agreement provides the foundation for resolving the conflict on Iran's nuclear program permanently," they wrote.

"This is why we now want to embark on the full implementation of the Joint Comprehensive Plan of Action, once all national procedures are complete."

The US Senate was due to vote later Thursday on a measure opposing the deal, after Republicans and some Democrats argued Iran can not be trusted to rein in its nuclear program and does not deserve sanctions relief.

But the resolution requires 60 votes to advance in the 100-member body, and the White House believes it has recruited enough Democratic support to block the legislation from moving forward.

Nevertheless, the European leaders' support will be welcome to Obama, who has seen what he hoped would be seen as a landmark diplomatic achievement and a victory for non-proliferation come under harsh attack.

"The long history of fruitless nuclear talks with Iran did not give strong grounds for optimism," they wrote.

"Nevertheless, two years of tough, detailed negotiation have produced an agreement that closes off all possible routes to an Iranian nuclear weapon in return for phased relief from nuclear-related sanctions.

"We fully support this agreement because it achieves the goals we had set ourselves," they added, arguing the deal will cut Iran's stock of enriched uranium and centrifuges and allows US inspectors into nuclear sites.