The United States used U.S.-Israeli spy software to hack into the French presidential office earlier this year, the French cyber-warfare agency has concluded, according to the newsmagazine l'Express.

The magazine reported late Tuesday that the computers of several close advisers to then-President Nicolas Sarkozy — including Chief of Staff Xavier Musca — were compromised in May by a computer virus that bears the hallmarks of Flame, which was allegedly created by a U.S.-Israeli team to target Iran's nuclear program. Anonymous French officials pointed the finger at the United States.

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“You can be on very good terms with a 'friendly' country and still want to guarantee their unwavering support — especially during a transition period,” an official told the magazine. The alleged spying attack took place a few days before the second round of the French presidential elections, which Sarkozy lost to Francois Hollande, a socialist.

The Obama administration on Wednesday, though, denied those reports.

“We categorically deny the allegations by unnamed sources that the U.S. government participated in a cyberattack against the French government,” Homeland Security spokesman Matthew Chandler told The Hill in a statement. “France is one of our strongest allies.

“Our outstanding cooperation in intelligence sharing, law enforcement and cyber defense has never been stronger, and remains essential in successfully combating the common threat of extremism,” Chandler added.

According to the l'Express report, Homeland Security Secretary Janet Napolitano reportedly did not deny the allegations when asked point-blank about them.

“We have no greater partner than France; we have no greater ally than France,” Napolitano reportedly answered, at the opening of an interview with l'Express. “We cooperate in many security-related areas. I am here to further reinforce those ties and create new ones.”

But an Obama administration official says Napolitano dismissed the question out of hand with laughter because it was “preposterous.”

In the interview, Napolitano also said that the Flame and Stuxnet viruses had “never been linked to the U.S. government.”

Parts of this article have been translated from the French.

This story was updated on Nov. 21 at 9:01 a.m. to include reaction from the Department of Homeland Security

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