Obama spoke by phone with his French counterpart Francois Hollande to assure him the US was no longer spying on European leaders, a day after the WikiLeaks website published documents alleging Washington had eavesdropped on the French president and his two predecessors.
"President Obama reiterated without ambiguity his firm commitment... to stop these practices that took place in the past and which were unacceptable between allies," Hollande's office said in a statement.
Hollande had earlier convened his top ministers and intelligence officials to discuss the revelations, with his office stating France "will not tolerate any acts that threaten its security".
France's foreign ministry also summoned the US ambassador for a formal explanation.
The documents labelled "Top Secret" and appearing to reveal spying on Jacques Chirac, Nicolas Sarkozy and Hollande between 2006 and 2012 were published by WikiLeaks along with French newspaper Liberation and the Mediapart website.
WikiLeaks' anti-secrecy campaigner Assange told French television late Wednesday the time had come to take legal action against Washington over its foreign surveillance activities.
Speaking on TF1, he urged France to go further than Germany by launching a "parliamentary inquiry" and referring "the matter to the prosecutor-general for prosecution".
German prosecutors had carried out a probe into alleged US tapping of Chancellor Angela Merkel's phone, but later dropped the investigation due to a lack of hard evidence.
Assange also said other important revelations were coming.
"This is the beginning of a series and I believe the most important of the material is still to come," he said.
The WikiLeaks revelations were embarrassingly timed for French lawmakers, who late on Wednesday voted in favour of sweeping new powers to spy on citizens.
The new law will allow authorities to spy on the digital and mobile communications of anyone linked to a "terrorist" inquiry without prior authorisation from a judge, and forces Internet service providers and phone companies to give up data upon request.
Addressing parliament, Prime Minister Manuel Valls said Washington's actions "constitute a very serious violation of the spirit of trust" and France would demand a new "code of conduct" on intelligence matters.
The White House earlier responded that it was not targeting Hollande's communications and will not do so in the future, but it did not comment on past activities.
Claims that the US National Security Agency (NSA) was spying on European leaders, revealed by whistleblower Edward Snowden in 2013, had already led to promises from Obama that the practice had stopped.
Secret meetings on Greece
The leaked documents include five from the NSA, the most recent dated May 22, 2012, just days after Hollande took office.
It claims Hollande "approved holding secret meetings in Paris to discuss the eurozone crisis, particularly the consequences of a Greek exit from the eurozone".
It also says the French president believed after talks with Merkel that she "had given up (on Greece) and was unwilling to budge".
"This made Hollande very worried for Greece and the Greek people, who might react by voting for an extremist party," according to the document.
Another document, dated 2008, was titled "Sarkozy sees himself as only one who can resolve the world financial crisis".
It said the former French leader "blamed many of the current economic problems on mistakes made by the US government, but believes that Washington is now heeding some of his advice".
One leak describes Sarkozy's frustration at US espionage, saying the "main sticking point" in achieving greater intelligence cooperation "is the US desire to continue spying on France".
But US officials vowed there was no spying going on now.
"Let me just be very, very clear ... we are not targeting President Hollande, we will not target friends like President Hollande," said US Secretary of State John Kerry.
"And we don't conduct any foreign intelligence surveillance activities unless there is some very specific and validated national security purpose."
Kerry told reporters he had "a terrific relationship" with his counterpart Foreign Minister Laurent Fabius, adding "the French are indispensable partners in so many ways" including in the Iran nuclear talks.
"The relationship between our two countries continues to get more productive and deeper," he added.
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