A family of seven are feared to have gone to join IS after going missing from their home in Bradford.
Imran Ameen, 39, his wife Farzana Ameen, 40, and their five children aged between five and 15 have not been seen since October 5.
Police believe they travelled to Turkey, but their current whereabouts are not known.
Officers from West Yorkshire Police are now working with relatives in the UK as well as Turkish authorities to try to track the family down.
Assistant Chief Constable Russ Foster said a "primary concern" was the safety and welfare of the young children, and to ensure the safe return of the family to Britain.
"We would urge anyone with information about the family’s whereabouts to come forward and speak to police so the family can safely return to the UK.
Any piece of information, no matter how small, could help the UK or overseas authorities to locate the family so that they can be safely returned home to their loved ones," Acc Russ Foster of West Yorkshire Police said.
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