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 on Tuesday.

France's election campaign commission said Saturday that "a significant amount of data" - and some information that was likely fake - was leaked on social networks following a hacking attack on centrist Emmanuel Macron's successful presidential campaign. France's government cyber-security agency is investigating what a government official described as a "very serious" breach.

The leak came 36 hours before the nation voted Sunday in a crucial presidential runoff between Macron and far-right candidate Marine Le Pen. The election commission said the leaked data apparently came from Macron's "information systems and mail accounts from some of his campaign managers" - a data theft that mimicked Russian hacking of the Democratic National Committee in the 2016 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?'" Adm. Mike Rogers told the Senate Armed Services Committee.

France's chief cyber-security body, known by the French acronym ANSII, declined comment on Rogers' testimony.

Earlier Tuesday, ANSII released a statement saying that it had been assisting with the response to the hack since Friday and that the information technology fraud division of Paris' police force had since been charged with investigating the breach.

Rogers also said 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, but "I would also tell myself, Rogers you are not moving fast enough."

He said the US needs to take tough actions against nation states working to undermine American democracy. They need to be publicly accused of the activity and the US needs to make it clear to them that this type of activity is "unacceptable and there is a price to pay for doing this."