The French president told Trump to “be serious,” delivering a real-time fact check of his false claim that most Islamic State fighters are from Europe.

Axar.az reports citing Fox News that President Donald Trump engaged in a public sparring match with French President Emmanuel Macron on Tuesday, offering to release detained members of the self-declared Islamic State into France.

Trump, who is meeting with world leaders in London this week at a NATO summit, said that the U.S. has “a tremendous amount of captured ISIS fighters over in Syria, and they’re all under lock and key,” falsely claiming they are “mostly from Europe.”