Story highlights UK police say the three girls, two age 15 and one age 16, are thought to be headed to Syria

They flew together Tuesday to Istanbul in Turkey, police say

London (CNN) UK police appealed for help Friday to find three teenage girls who are missing from their homes in London and are believed to be making their way to Syria.

The girls, two of them 15 and one 16, have not been seen since Tuesday, when, police say, they took a flight to Istanbul.

One has been named as Shamima Begum, 15, who may be traveling under the name of 17-year-old Aklima Begum, and a second as Kadiza Sultana, 16. The third girl has not been named at the request of her family, police said.

Pictures released by police show the three girls walking together through London Gatwick Airport, carrying travel bags. They are said to be close friends who went to the same east London school.

Richard Walton, of Counter Terrorism Command, said: "We are extremely concerned for the safety of these young girls and would urge anyone with information to come forward and speak to police. Our priority is the safe return of these girls to their families.

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