LONDON — Britain's prime minister has urged the nation's schools to be vigilant in the fight against Islamic extremism after three British schoolgirls left the country in a suspected bid to join the Islamic State group.

David Cameron said Saturday the teenagers' disappearance was "deeply concerning" and underlines how every school and college needs to recognize they have a role to play in stopping people from joining what he called an "appalling death cult."

The three girls, aged 15 to 16, flew to Turkey from London on Tuesday without leaving any messages behind.

Police said the force was becoming increasingly concerned about a growing trend of girls and young women showing an interest in joining the Islamic State group. Officials say hundreds of Britons have joined extremists in Syria.