In a statement released on Tuesday, British police said they thought Shamima Begum, 15, Amira Abase, 15, and Kadiza Sultana,16, had reached Syria. The three had left their London homes to fly to Istanbul, Turkey, on February 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, giving no further details except that they continued to work closely with Turkish authorities.

Authorities suspect the young women had been lured online into running away to join the "Islamic State" (IS) extremist group, which during the past year has claimed large parts of Iraq and Syria and declared it a Sunni Muslim caliphate to be ruled under its brutal interpretation of Sharia law.

"They appear to have been induced to join a terrorist group that carries out the most hideous violence and believes girls should be married at nine and women should not leave the home," British Prime Minister David Cameron said Monday.

Security forces estimate at least 500 Britons have traveled to the region to fight with extremist groups including IS.

The families of the three missing girls, who were described as high-achieving students, have implored them to come back home. A fourth girl from the same London school disappeared in December and was also feared to have gone to Syria. Police said detectives investigating that case spoke with the three girls at the time, but nothing suggested they were at risk of being radicalized.

Authorities trade blame

Turkey, which has in the past copped criticism over whether it is doing enough to stop prospective militants traveling through its territory, accused Britain of being too slow to share information about the teenagers, who entered Turkey on tourist visas.

"We don't have a mechanism that allows us to question or read the minds of tourists," Turkish Deputy Prime Minister and government spokesman Bulent Arinc told reporters in Ankara late Monday, after saying that it took British police three days to let Turkish authorities know of their concerns for the girls.

However, London's Metropolitan Police disputed that account, saying in a statement Tuesday that the foreign liaison officer at the Turkish embassy in London was contacted the day after the three went missing.

The case has also prompted calls for social media firms to do more to deal with extremism online, as well as tighter controls by airlines for children and teenagers traveling unaccompanied.

se/mg (AP, AFP, Reuters)