The case of three girls between the ages of 15 and 16 from the U.K. who travelled to Turkey to join the Islamic State of Iraq and al-Sham in Syria has triggered a debate over whose responsibility it is to stop the flow of people attempting to cross into Syria from Turkey to join ISIS.

Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16, boarded a Turkish Airlines flight to Istanbul from London's Gatwick Airport on Feb. 17. They went missing once they arrived in Turkey. Turkish and British media outlets claimed the three girls, who attend Bethnal Green Academy in London, crossed into Syria to join ISIS. It was revealed that Begum had contacted Aqsa Mahmood, a woman from Glasgow who left for Syria in 2013 to marry an ISIS member and the girls had contact on social media with "extremist" accounts.

As the case has generated attention both in the U.K. and Turkey, British Prime Minister David Cameron flatly piled the blame on Turkish Airlines, which admitted the girls aboard one of its flights. It was reported that one of the girls was traveling with the passport of her 17-year-old sister.

Cameron told parliament on Monday: "What this incident has highlighted is the concerning situation where unaccompanied teenagers like these – who are not a known risk – can board a flight to Turkey without necessarily being asked questions by the airline." Cameron pointed to the Border Warnings Index that enables security services to intervene to prevent travel of people and seizure of their passports.

Social media companies were also targeted by Cameron for not doing more on the matter of online extremism. "All of us have been horrified by the way that British teenagers appear to have been radicalized and duped by this poisonous ideology of Islamist extremism while at home on the internet in their bedrooms," Cameron said. "Given reports that one of the girls was following as many as 70 extremists online, this case underlines the importance of the work we are doing with social media companies," he added. He said Internet companies had a social responsibilities and they are expected "to live up to it."

Cameron also said they exerted efforts for British security services to have access to passenger name records for "as many routes as possible in and out of Britain."

Turkish Airlines did not disclose whether the girls were admitted to fly to Istanbul citing privacy laws and denied any wrongdoing in allowing people of such age to board.

Officials for the carrier said all flights and passengers allowed to fly were held in compliance with international and U.K. standards. They said people aged 13 and over can fly unaccompanied and they were subject to passport checks and other security checks that are entirely carried out by British authorities before boarding any flight. The carrier said in a statement that Turkish Airlines "had the responsibility to control the validity of the required visas of its passengers for international travel during the check-in and boarding process at airports before the flight. All the additional passenger security protocols before a flight departs are the responsibility of the official airport authorities."

Another point of contention between Turkey and Britain was the notification of Turkish security forces on the missing trio. Deputy Prime Minister Bülent Arınç said on Monday that British authorities notified them about the girls three days after their flight landed in Istanbul. "Turkey cannot be blamed. We receive millions of tourists every year and although we have every measure in place against people seeking to join terrorist organizations in another country, sharing of intelligence is vital to stop the flow. They have Scotland Yard, a world-renowned organization, yet they did not monitor these people despite earlier suspicions. It is a shame, a condemnable act that they noticed the girls headed to Turkey freely," Arınç said. He said Turkey cannot question the motives of people who might have been tourists as well. "We cannot read minds," he said.

The London Metropolitan Police released a statement on Tuesday and said: "Once we established that the girls had traveled to Turkey, police made contact with the foreign liaison officer at the Turkish Embassy in London on Wednesday, Feb. 18."

The British government announced earlier that some 600 citizens are "estimated" to be traveling to Syria and Iraq to join ISIS. More than 200 people were detained last year for having connections with ISIS.

Cameron did not say what his country has been doing to curb the "radicalization" of its citizens in the face of ISIS's recruitment attempts in the U.K. and continental Europe.

Mark Keary, head teacher of the school where the three girls attend, told reporters that there was no evidence that the girls were at risk of being radicalized and the police were not concerned about their possible radicalization although they were questioned following a friend of the girls being suspected of traveling to Syria last year.

ISIS draws considerable popularity from impressionable and disillusioned youth in Europe, according to experts. Countries where militant hopefuls hail from are criticized for a lack of measures against ISIS recruitment and focusing only on military actions against the armed faction that has captured wide swathes of Syria and Iraq.

Turkey has complained of a lack of intelligence cooperation from countries whose citizens have sought to join ISIS by crossing through Turkey to Syria. Earlier this month, 14 foreigners were apprehended by security forces as they attempted to cross the border with Syria. According to official figures, 1,400 people from several countries were deported for attempting to cross into Syria thus far and the country has banned 10,000 people from 91 countries from entering Turkey.

The country is the main route for would-be ISIS militants due to its long border with Syria, and the country often faces criticism for failing to curb the flow of foreigners. Turkish authorities, however, dismiss the criticism and point to ramped-up controls along the 911-kilometer border that is heavily manned by troops, especially after the conflict broke up in Syria in 2011. Sharing intelligence is vital to stop the flow of fighters, according to Turkish authorities, but intelligence blunders on the part of the militants' home countries have challenged them.

One such intelligence failure was that of Hayat Boumedienne, an accomplice and widow of a gunman who stormed a kosher supermarket in Paris on Jan. 8. Boumedienne had traveled to Istanbul where she stayed at a hotel five days before the attack and crossed into Syria from there. She had traveled to Turkey from Spain and the Turkish interior minister said that French intelligence did not inform them about the woman.