Missing UK teens seen on video in Turkey before going to Syria

(CNN)Three teen British girls suspected of traveling to Syria appeared on surveillance video in Turkey before they went to their destination.

Images show the teens boarding a bus in Istanbul, Turkish HBR television reported. It's the first sighting of the girls since they left London almost two weeks ago.

Authorities have said they have no reason to believe the girls are still in Turkey and believe they have crossed into neighboring Syria, parts of which have been taken over by Islamist terror group ISIS.

The three east London classmates -- Shamima Begum, 15; Kadiza Sultana, 16; and Amira Abase, 15 -- boarded a Turkish Airlines plane from London's Gatwick Airport to Istanbul on February 17.

A British counterterrorism officer said police had tried to reach out to the girls using the Turkish media and social media to persuade them to return home.

The girls' parents have also publicly begged for them to come home.

Reaching out to ISIS recruiter

Days before they left for Turkey, at least one of the girls allegedly contacted a young woman, Aqsa Mahmood, who left her home in Scotland to travel to Syria in 2013 and is accused of trying to recruit others via social media.

An attorney for the Mahmood family said the family was "surprised, horrified" that the contact had not prompted police action "because they understood that Aqsa's social media content was being closely monitored by the security services."

"And the real concern that arises is that if a known member of ISIS who's now a poster girl for ISIS, seen to be recruiting and spreading propaganda, is in touch with young people in our country -- that one would expect the courtesy of a knock on the door of these families, to advise them that their children may be on the cusp of radicalization or going off to Syria," Aamer Anwar said.

The Mahmood family condemned their daughter's actions, describing them "as a perverted and evil distortion" of Islam.

But they also said the UK security services "have serious questions to answer" over their failure to intervene after one or more of the missing girls got in touch with her.

A popular gateway

Turkey has become the most traveled route for volunteers trying to join Syria's many armed factions since an anti-government uprising erupted more than three years ago.

Initially, Ankara maintained an "open door policy," allowing hundreds of thousands of Syrian refugees to flee to safety in Turkey.

Turkish border guards looked the other way, taking few steps to stop activists, volunteers and journalists from crossing into Syria.

But with the rise of the ultra-violent Islamist group ISIS in Syria, the Turkish government increasingly came under criticism at home and abroad for not doing enough to stop jihadists from going through Turkey to get into Syria.

Some critics have accused Ankara of actively supporting ISIS against rival Kurdish rebel groups, charges the Turkish government has angrily denied.

Last year, the Turkish government showed CNN new barricades, thermal cameras and fences aimed at better fortifying the border.

Historically, the Turkish-Syrian border, which runs more than 800 kilometers (500 miles), has been notoriously difficult for Turkish security forces to control. Before the Syrian civil war, both smugglers and Kurdish separatist guerrillas were known to operate illegally between the two countries.

Turkish government officials have bristled in response to accusations that they are not stemming the flow of foreign fighters joining ISIS.

Millions of tourists visit Turkey every year, where visas can be obtained upon arrival for citizens of many countries.