Three British sisters are feared to have travelled to Syria with their nine children to join up with extremists.
Police have launched an investigation after members of the Dawood family, from Bradford in West Yorkshire, went missing after going on an Islamic pilgrimage to Saudi Arabia.
The news emerged as calls mounted for more to be done to tackle radicalisation of teenagers online after 17-year-old Talha Asmal was reported to have become Britain's youngest ever suicide bomber.
While Thomas Evans, a Muslim convert from Buckinghamshire, is believed to have died fighting in Kenya fighting for extremist group Al Shabaab.
Sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, travelled to Medina in Saudi Arabia with their nine children, aged three to 15, on May 28 to go on a religious pilgrimage.
They were due to return to the UK on June 11, but they broke off all contact with their family back in Britain two days earlier on June 9.
It is believed at least 10 members of the family boarded a flight from Medina to Istanbul in Turkey - a commonly used route into Syria.
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 or another extremist group in Sy ria, 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."
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 the sisters and their children have not been in contact for nearly a week, their mobile phones have been turned off, and their Facebook profiles have not been updated.
He said: "It started with a pilgrimage to Saudi Arabia and they were meant to return on June 11, however they have failed to return.
"It has been revealed that on June 9 they have travelled from Medina to Istanbul. Since that day their family have not had any contact with them and their Facebook and Whatsapp profiles have not been updated.
"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."
Preliminary inquiries suggest that 10 members of the family boarded the flight to Istanbul. No details have been found for Haafiyah Zubair and Nurah Zubair boarding that flight.
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.
Speaking after a meeting with West Yorkshire Police, Mr Khan told BBC News the fathers of the missing children are "quite distraught".
Asked if there was real concern for the safety of the missing Dawoods, he said: "Yes there is at this stage because there has been no contact with them from 9 June, unfortunately we can only fear the worst. So there is grave concern that the families share."
He said it is unclear where the family went after they landed in Istanbul and the fathers have a lot of "unanswered questions", but hopefully the Turkish authorities will be able to shed more light on their movements.
He said: "I'm extremely concerned on behalf of the family members, I share their concerns - it is young children that have gone."
A West Yorkshire Police spokeswoman said it has been supporting the family, adding that officers have started an investigation to establish their whereabouts and are "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."
Talha, 17, is alleged to have fled his home in Dewsbury, West Yorkshire, in March, to join IS - also known as Isis or Isil.
He reportedly detonated a vehicle fitted with explosives while fighting for the group in Iraq.
His 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.
"The perpetrators are pretty much acting like paedophiles, they groom these young individuals over time - radicalisation isn't an overnight process," he told the BBC.
"They prey on these vulnerable young people and brainwash them, and religion is a unique passion so they tend to use religion to brainwash these young individuals for their own political aims and gains."
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 today of the varied nature of the threat faced by the UK.
Speaking ahead of a police counter-terrorism conference, 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."
Unbeknown to them and completely against their will, Talha's family said he travelled to Iraq via Turkey and fell under the spell of Isis handlers who are "too cowardly to do their own dirty work".
Shahid Malik, a former government minister and a family friend of the Asmals, said: " This is a clear indication of just how successful the evil Isis groomers have been in poisoning and brainwashing Talha and kids like him."
Iqbal Bhana, a member of a cross-governmental anti-Muslim hate crime advisory group, said h e knew the family of 17-year-old Hassan Munshi, who is believed to have travelled to Syria from Dewsbury with Talha.
He said Hassan's family were "devastated" at the news about Talha and hoped it might make Hassan come home.
He said: "They were hoping against hope and the community has been praying since they went in the hope that they would come back."