Three British sisters are feared to have travelled to war-torn Syria with their children, who are aged as young as three years old.
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.
It was also revealed today that Thomas Evans, a Muslim convert from Buckinghamshire, is believed to have died fighting in Kenya fighting for extremist group Al Shabaab.
It is feared 12 members of the Dawood family, from Bradford in West Yorkshire, travelled to Syria where it is understood one of their relatives is fighting for extremists.
According to Balaal Khan, a lawyer acting for the fathers of the missing children, sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, left Britain on May 28 to go on an Islamic pilgrimage to Saudi Arabia.
They travelled with their nine children and were supposed to return to the UK on June 11.
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.
Their family have not heard from them in almost a week, and it is believed that on June 9 they boarded a flight from Madina in Saudi Arabia to Istanbul in Turkey – a commonly used route into Syria.
Mr Khan 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 said the sisters and their children have not been in contact for nearly a week, 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 Madina 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."
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 confirmed they have received a missing persons report concerning the Dawood family.
The force said that since their disappearance was reported 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."