The United States watched Russians hack France's computer networks during the election and tipped off French officials before it became public, a US cyber official told the Senate today.
France's election campaign commission said on Saturday that "a significant amount of data" - and some fake information - was leaked on social networks following a hacking attack on centrist Emmanuel Macron's successful presidential campaign.
France's government cybersecurity agency is investigating what a government official described as a "very serious" breach.
The leak came 36 hours before the nation voted in a crucial presidential run-off between Mr Macron and far-right candidate Marine Le Pen.
The election commission said the leaked data apparently came from Mr Macron's "information systems and mail accounts from some of his campaign managers" - an attack that mimicked Russian hacking of the Democratic National Committee in last year's US presidential election.
"We had become aware of Russian activity.
"We had talked to our French counterparts and gave them a heads-ups - 'Look, we're watching the Russians. We're seeing them penetrate some of your infrastructure. Here's what we've seen. What can we do to try to assist?'" Admiral Mike Rogers told the Senate Armed Services Committee.
He said the US is also working closely with German and British counterparts.
Mr Rogers added that the US is still working on a comprehensive cyber policy to counter what he called a "brave new world" in the cyber domain.
He said the United States is improving its ability to defend itself against cyberattacks, but "I would also tell myself, Rogers you are not moving fast enough".