Friends Amira Abase, 15, Shamima Begum, 15, and Kadiza Sultana, 16, flew to Istanbul from London on Feb. 17.

"Officers ... leading the investigation into the three missing schoolgirls from east London, now have reason to believe that they are no longer in Turkey and have crossed into Syria," police said in a statement.

The girls' plight has prompted widespread concern in Britain, with Prime Minister David Cameron urging social media firms to do more to deal with online extremism, saying the girls appeared to have been radicalised "in their bedrooms".

The three are friends with a fourth teenage girl from the same school who police believe is already in Syria, having travelled to Turkey in December.

Police said their families, who have issued urgent appeals for their daughters to return, have been surprised and devastated by their disappearance.

Security forces estimate some 600 British Muslims have travelled to the region to join the conflict there, some of them with ISIS.

Around half have since returned, and dozens have been arrested in Britain under anti-terrorism legislation.