Media playback is unsupported on your device Media caption Ali's father believes he has joined Isis to fight in Syria, as the BBC's Shaimaa Khalil reports

The father of a British teenager who travelled to Syria to join jihadists believes his son was radicalised by an imam at a UK mosque.

Rahim Kalantar told the BBC his son Ali, 18, travelled to Syria with two friends from Coventry in March and believed he was now fighting with Isis.

He said he was sent "down this road" by an imam - who denied the allegations.

Up to 500 Britons are thought to have travelled to the Middle East to fight in the conflict, officials say.

Mr Kalantar - speaking to BBC Two's Newsnight, in collaboration with the BBC's Afghan Service and Newsday - said he worries about his son Ali "every minute" and that his grief is "limitless".

Image copyright AFP Image caption Around 500 Britons are believed to have travelled to Syria, where Isis militants are involved in fighting

He said he believed Ali - who was planning to study computer science at university - had been radicalised during classes at a mosque after evening prayer.

"He [the imam] encouraged them and sent them down this road," he said.

The BBC contacted the mosque to speak to the imam, who refused to give an interview but said he completely denied the allegations.

Ali is believed to have travelled to Syria with Rashed Amani, also 18, who had been studying business at Coventry University.

Rashed's father, Khabir, said family members had travelled to the Turkish-Syrian border in the hope of finding the boys, but came back "empty-handed" after searching for more than two weeks.

He said he did not know what had happened to his son, who he fears has joined Isis - the militant-led group that has made rapid advances through Iraq in recent weeks.

"Maybe somebody worked with him, I don't know. Maybe somebody brainwashed him because he was not like that," he said.

The third teenager, Moh Ismael, is also believed to be in Syria with his friends. He is understood to have posted a message on Twitter saying he was with Isis.

Image caption Nasser Muthana (r) features in an apparent recruitment video. His brother Aseel (l) was said by their father to have also travelled to Syria

It comes after Britons - including Reyaad Khan and Nasser Muthana from Cardiff - featured in an apparent recruitment video for jihadists in Iraq and Syria.

The video was posted online on Friday by accounts with links to Isis.

Social media

The BBC has learned a third Briton in the video is from Aberdeen. The man, named locally as Raqib, grew up in Scotland but was originally from Bangladesh.

Lord Carlile, a former independent reviewer of terrorism laws, told the BBC that the Muslim community was best placed to stop jihadists recruiting in the UK.

The Liberal Democrat peer also said the UK needed to reintroduce tougher measures to stop terrorism.

It comes after former MI6 director, Richard Barrett, said security services would not be able to track all Britons who return to the UK after fighting in Syria.

He said the number of those posing a threat would be small but unpredictable.

The Metropolitan Police has insisted it has the tools to monitor British jihadists returning from that country.

Shiraz Maher, a radicalisation expert, told Newsnight that social media was now acting as a recruitment ground for potential jihadists in the UK.

Media playback is unsupported on your device Media caption This footage purports to show Reyaad Khan (C) and Nasser Muthana (R) from Wales

"You have hundreds of foreign fighters on the ground who in real time are giving you a live feed of what is happening and they are engaged in a conversation.

"It is these individual people who have been empowered to become recruiters in their own right," he said.

Lord Carlile said the "most important partners" in preventing young Muslims from being radicalised were the "Muslim communities themselves".

"Mothers, wives, sisters do not want their husbands, brothers, sons to become valid jihadists and run the risk of being killed in a civil war," he told the programme.

He also told BBC Radio 4's World at One programme that the government should look at reintroducing "something like control orders", which were scrapped in 2011 and replaced with the less restrictive Terrorism Prevention and Investigation Measures (TPims).

He said: "We need to look at preventing violent extremism before people leave the country and also we need to look for further measures."