For years, France has been viewed as the toughest member of the group of powers known as the P5+1, after feeling burned in previous pacts under which Tehran covertly continued to advance its atomic ambitions.
The P5+1 -- Britain, China, France, Russia, the United States and Germany -- go back to the negotiating table next week in Geneva having failed to meet a November 24 deadline for a deal.
They have set a new target date of June 30 to reach a pact that would end 12 years of protracted negotiations with the Islamic republic.
But despite public assertions of unity among the global powers, Western diplomats confirm there is a diplomatic fencing match behind the scenes between Paris and Washington.
One of France's main concerns is the incomplete Arak heavy water reactor, which when it eventually comes online could be used to make plutonium for an atomic bomb.
Paris is said to have pushed for stringent inspections of Iran's nuclear energy program, and a broad dismantling of facilities and centrifuges.
Amid great uncertainty over whether a deal is possible despite hours and hours of tense negotiations, this distrust between the two transatlantic allies could prove the weakest link in the P5+1 bloc.
Privately, American officials say there has been concern in Washington over the French position of publicly playing hardball, but then not backing up their words in the negotiations.
In November 2013, angered by US Secretary of State John Kerry's sudden appearance at the talks in Geneva, French Foreign Minister Laurent Fabius showed up, talking tough against the proposed contours of a deal and seemingly threw a wrench in the works.