LONDON (CNN) -- French Foreign Minister Laurent Fabius met Tuesday morning with U.S. Secretary of State John Kerry to discuss allegations that the National Security Agency intercepted more than 70 million phone calls in France over a 30-day period.

Fabius called the practice unacceptable and told Kerry that it must stop, the French Foreign Ministry said in a statement. The pair also discussed the situation in Syria ahead of a "Friends of Syria" meeting that's taking place in London on Tuesday.

The top diplomats huddled a day after the details of the alleged spying appeared in the French newspaper Le Monde.

U.S. President Barack Obama and French President Francois Hollande spoke Monday on the matter, according to a White House statement.

"The President and President Hollande discussed recent disclosures in the press -- some of which have distorted our activities and some of which raise legitimate questions for our friends and allies about how these capabilities are employed," the statement said. "The President made clear that the United States has begun to review the way that we gather intelligence, so that we properly balance the legitimate security concerns of our citizens and allies with the privacy concerns that all people share."

A news release from Hollande's office said he expressed his "deep disapproval with regard to these practices" to Obama and that such alleged activities would be unacceptable between allies and friends.

The two presidents agreed that French and American intelligence services will cooperate on investigating the report.

Intercepting millions of calls

The NSA monitored the phone calls made in France, Le Monde reported Monday, citing documents leaked by former NSA contractor Edward Snowden.

The intercepts took place from December 10, 2012, to January 8, 2013, the article reported. An NSA graph shows an average of 3 million data intercepts a day.

According to Le Monde, this is how the system worked: "When a telephone number is used in France, it activates a signal which automatically triggers the recording of the call. Apparently this surveillance system also picks up SMS (text) messages and their content using key words. Finally, the NSA apparently stores the history of the connections of each target -- or the meta-data."

It wasn't immediately clear from the article if the conversations were recorded or just the data surrounding each call.

Other spying allegations

The report follows weekend article in the German news magazine Der Spiegel that said the NSA "systematically" eavesdropped on the Mexican government. It hacked the public e-mail account of former Mexican President Felipe Calderon, which was also used by Cabinet members, according to Der Spiegel.

The magazine also quoted documents leaked by Snowden.

"This practice is unacceptable, illegitimate and against Mexican and international law," Mexico's Foreign Ministry said in a statement.

It added that it would push for a speedy investigation.

"In a relationship between neighbors and partners, there is no room for the practices alleged to have taken place," the ministry said.

A senior U.S. State Department official told CNN that the Mexican government reached out about the report and that the two governments will be discussing it via diplomatic channels.

In September, Mexico and Brazil summoned U.S. ambassadors after media reports that the United States had spied on their countries' presidents. Those reports were also based on documents leaked by Snowden.