Fears are growing for the safety of three British sisters believed to have travelled to Syria with their nine children to join up with extremists.
Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, whose children are aged between three and 15, went missing after going on an Islamic pilgrimage to Saudi Arabia from their homes in Bradford, West Yorkshire.
The children are believed to have flown with their mothers from Medina in Saudi Arabia to Istanbul, Turkey - a commonly used route into Syria.
Community leaders called for more to be done to tackle the radicalisation of teenagers online after 17-year-old Talha Asmal, from Dewsbury, West Yorkshire, was reported to have become Britain's youngest suicide bomber.
And Thomas Evans, a Muslim convert from Buckinghamshire, is believed to have died in Kenya fighting for extremist group Al Shabaab.
The Dawood sisters travelled to Medina with their nine children on May 28 to go on a religious pilgrimage.
They were due to return to the UK on June 11, but broke off all contact with their family in Britain two days earlier on June 9.
Preliminary inquiries suggest that at least 10 members of the family boarded the flight to Turkey that day.
Since then, the family's mobile phones have been turned off and Facebook and WhatsApp profiles have not been updated.
Balaal Khan, a lawyer acting for the fathers of the missing children, said it is understood the sisters have a relative fighting for either Islamic State (IS) or another extremist group in Syria, and it is feared they have met up with him.
He said: "They are concerned that their children's lives are in danger.
"The concern is for the well-being and safety of the children. The fathers are distraught, they feel helpless and they don't now what to do. They want the children out of harm's way."
He added: "One of the possibilities is they travelled to Turkey to travel to Syria. The suspicion, and main concern, is that the women have taken their children to Syria."
According to Mr Khan, the missing children are five-year-old Muhammad Haseeb; Maryam Siddiqui, seven; Ismaeel Iqbal, three; Mariya Iqbal, five; Zaynab Iqbal, eight; Ibrahim Iqbal, 14; Junaid Ahmed Iqbal, 15; Haafiyah Binte Zubair, eight; and five-year-old Nurah Binte Zubair.
He said police were notified five or six days ago, but are limited in what they can do because it is out of the jurisdiction.
A West Yorkshire Police spokeswoman said the force had been supporting the family, adding that officers started an investigation to establish their whereabouts and were "working extensively with authorities overseas to try and locate them".
Assistant Chief Constable Russ Foster said: "We are extremely concerned for the safety of the family and would urge anyone with information to come forward and speak to us.
"Our priority is for their safe return; their families are gravely worried about them and want them home. One of our primary concerns is the safety and welfare of the young children."
A Foreign Office spokeswoman said: "We are in contact with West Yorkshire Police and Turkish authorities and our ready to provide consular assistance."
In Dewsbury, people expressed their shock after the news emerged that Talha, who was alleged to have fled his home in March to join IS, reportedly detonated a vehicle fitted with explosives while fighting for the group in Iraq.
The teenager's family said he had been exploited by extremists on the internet "in a process of deliberate and calculated grooming".
Qari Asim, an imam at the Makkah Mosque in Leeds, said recruitment was mainly taking place online.
He said: "Isis have been running a very sophisticated social media campaign in order to showcase a romantic, utopian world that they are establishing in Iraq."
He added: "We need to really up our game, reclaim some of the cyberspace and that's what Imams Online are doing, imams from across the country have come together to really reclaim some of that space online by showcasing the true reality of Isis and also really sending a positive message about the true teachings of Islam."
However, Mr Asim said radicalisation was not isolated to the internet.
He said: "The majority of radicalisation and brainwashing takes place online but I think it would be wrong to say actually it doesn't take place offline because it's pretty much like paedophiles, they will use all sorts of opportunities to their avail to groom people."
David Cameron's official spokeswoman said the Prime Minister is "clear that the case is deeply concerning".
The former reviewer of anti-terrorism legislation, Lord Carlile, called on the Government to work with computer programmers and social media companies to counter extremist propaganda online.
"It's hard to counter, but one does have to use the same tools, the same thought processes, that do radicalise people," said the Liberal Democrat peer.
Sir Bernard Hogan-Howe, the country's most senior police officer, spoke of the varied nature of the threat faced by the UK.
The Metropolitan Police commissioner said: "Today's terrorist may of course be a hardened member of an organised terrorist 'cell', but may very well also be a lone disaffected youth radicalised by extremist material on his home computer.
"Some of those travelling to Syria are fulfilling a long-standing jihadi ambition. But others who travel to Syria are youngsters fooled by propaganda - out of their depth and running out of time.
"The police must find a way to deal with both."