The French president, Francois Hollande, condemned US spying on French citizens in a call with President Barack Obama, as a row escalated over US eavesdropping on millions of French phone calls.
Hollande's office said in a statement that he had expressed "deep disapproval of these practices, which are unacceptable between friends and allies because they infringe on the privacy of French citizens".
Hollande "asked that all explanations be provided, as well as all information that could be at the disposal of former NSA consultant Edward Snowden".
The statement said the two leaders had agreed "to work together to determine the facts and the exact scope of surveillance activities" revealed by French newspaper Le Monde.
The two stressed that surveillance operations should be put into a "bilateral framework" and agreed that US and French intelligence agencies would "work together to this effect", the statement said.
The spying claims threatened to overshadow a visit to Paris by the US secretary of state, John Kerry, who is due to discuss the war in Syria with western allies and members of the opposition.
Millions of calls
Reports in Le Monde and the German Der Spiegel have revealed that the National Security Agency secretly recorded tens of millions of phone calls in France, and hacked into former Mexican President Felipe Calderon's email account.
The spy agency taped 70.3 million phone calls in France over a 30-day period between December 10 and January 8 this year, Le Monde reported in its online version, citing documents from Snowden.
According to Le Monde, the NSA automatically picked up communications from certain phone numbers in France and recorded text messages under a programme code-named "US-985D".
In July, Paris prosecutors opened a preliminary inquiries into the NSA's programme, known as Prism, after Der Spiegel and Britain's The Guardian revealed wide-scale spying by the agency leaked by Snowden.
"We were warned in June [about the programme] and we reacted strongly but obviously we need to go further," said the French foreign minister, Laurent Fabius. "We must quickly assure that these practices aren't repeated."