David Cameron says the disappearance of three east London girls feared to be heading to Syria to join Islamic State extremists is "deeply concerning".
Shamima Begum, 15, Kadiza Sultana, 16, and an unnamed 15-year-old friend, flew from Gatwick to Turkey on Tuesday.
Police, who interviewed the girls after another girl from their school went to Syria in December, said the development had come as a "great surprise".
They said nothing at the time had indicated the three girls were at risk.