Syrian rebels, including the al-Qaeda linked al-Nusrah Front, claim to have seized control of Idlib - capital of Syria's northwestern province.
Nine British medical students and doctors are feared to have travelled to Syria to work in areas controlled by Islamic State militants.
Two of the London teenagers stopped in Turkey at the weekend were heading to Syria "to do some good", says Mohammed Butt, leader of Brent Council, who has spoken to the boys and their families.
The UK's emphasis on Islamist terrorism has meant it has ignored the traditional threat from nation states like Russia, according to a former head of MI6.
Sabiha Gokcen airport releases footage of three teenagers - two cousins aged 17 and a man aged 19 - from London being detained as they allegedly attempt to travel to Syria to join Islamic State.
Graffiti sprayed last year on the ruins of Cyrene, in Libya, reads: "Destroy the Idols". Islamists, including Islamic State militants, see such pre-Islamic structures as sacrilege.
A 15-year-old, 17-year-old and 19-year-old from north-west London arrested at Istanbul airport and returned to UK, while an 18-year-old is arrested in Birmingham on suspicion of travelling to Syria.
Three British teenagers who were stopped in Turkey on their way to Syria are back in the UK and are being held at a central London police station, Scotland Yard says.
The father of a British teenager who fled to Syria to join Islamic State militants before three school friends from east London warned police to monitor them.