Three British women and their nine children are believed to have travelled from Turkey into Syria to join extremists fighting for Islamic State (Isis) or another militant group.
West Yorkshire police, which has launched an investigation into the missing Dawood family, has confirmed that nine children between the ages of 3-15 years are with the group and are travelling with their mothers, three sisters: Khadija, Sugra and Zohra Dawood.
The family's disappearance was reported to police on 11 June after they went missing following a pilgrimage to the Muslim holy city of Medina in Saudia Arabia. It is believed at least 10 members of the family boarded a flight from Saudi to Istanbul, a regular stopping point for those wishing to pass into Syria over the Turkish border.
Assistant Chief Constable Russ Foster, of West Yorkshire Police, said in a statement: "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."
Lawyer Balaal Khan, speaking on behalf of the father of the nine missing children, has said it is believed that Khadija, Sugra and Zohra have gone to join a male relative who has been fighting for IS or another Jihadi group in Syria.
He said: "[The fathers] 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 was quoted by the Press Association as saying.
According to Mr Khan the missing children are Muhammad Haseeb (5), Maryam Siddiqui (7), Ismaeel Iqbal (3), Mariya Iqbal (5), Zaynab Iqbal (8), Ibrahim Iqbal (14), Junaid Ahmed Iqbal (15), Haafiyah Binte Zubair (8), and Nurah Binte Zubair (5).
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.
The Foreign Office is in contact with West Yorkshire Police and Turkish authorities and is prepared to provide consular assistance.
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. The teenager reportedly killed himself in a suicide bombing in Iraq.