The Metropolitan Police said the officers have "reason to believe" the three teenagers from Bethnal Green Academy in east London, have joined the Takfiri terrorists in Syria.
Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, boarded a flight from London Gatwick airport to Turkey last Tuesday.
UK police have been under fire for delay in informing the Turkish authorities about the teenagers travel.
Some reports suggest that Scotland Yard waited three days before telling the Turkish authorities that the teenagers had flown to Istanbul.
Turkey's Deputy Prime Minister Bulent Arinc said that the British authorities notified Ankara about the missing schoolgirls on Friday.
Turkish officials would have taken "necessary measures" had they known earlier, said Arinc, arguing that the UK would be partly responsible for the girls’ fate.