Police investigating the whereabouts of three British schoolgirls after they flew to Istanbul have said they believe the teenagers are no longer in Turkey and have crossed into Syria.

The girls, aged 15 to 16, disappeared from their London homes on February 17 and boarded a flight to Istanbul.

Authorities said on Tuesday that they believe the girls could be on their way to join the Islamic State of Iraq and the Levant (ISIL).

The case underlines fears that growing numbers of young women in Britain and Europe are being lured by online propaganda to join ISIL.

Security officials say at least 500 Britons have travelled to Syria to fight alongside ISIL, often via Turkey. Experts estimate about 50 are female.

The three girls in the latest case have all been described as "straight-A students" from a highly-regarded London school, according to the Associated Press.

The families of Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, have appealed for them to return home.

"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," Prime Minister David Cameron said on Monday.

UK 'lost valuable time'

Turkey's deputy prime minister has complained that British officials waited three days before seeking help in the case, losing valuable time.

Bulent Arinc said the girls arrived in Istanbul as tourists, and UK authorities did not share enough information for Turkey to act quickly.

"It is a condemnable act, a shameful act that a country like Britain ... did not follow [the girls] closely," Arinc told reporters in Ankara, the capital. "They woke up three days after the fact to notify us."

"We don't have a mechanism that allows us to question or read the minds of tourists."

The Metropolitan Police disputed that account, however, saying on Tuesday that they notified the Turkish embassy in London a day after the girls went missing.

Police from both countries are working together to investigate the case, the force added.

A fourth girl from the school where the missing girls studied disappeared in December and was thought to have left for Syria.

Police said detectives investigating that case spoke to the three girls at the time, but there was nothing to suggest they were at risk of radicalisation.

Source: Associated Press