Twitter says it has suspended 360,000 user accounts in the past 18 months for threatening or promoting acts of terrorism.

Lawyers for the families of terror victims, including a brother and sister killed in last year's bomb attacks in Brussels and an American college student slaughtered in Paris, say that isn't enough.

In a string of lawsuits filed in New York, they say they want Twitter and Facebook to pay damages for failing to stop violent extremists from using their platforms to recruit followers.

The companies say the lawsuits get it wrong. Facebook says it has zero tolerance for terrorism and swiftly removes any terrorism content. Twitter says no terrorist-related action or violent threat is permitted on its platform.