Two West Yorkshire teenagers are feared to have travelled to Syria, police have said.
The 17-year-old Dewsbury boys are thought to have flown from Manchester to Dalaman in Turkey on 31 March before making their way to Syria.
West Yorkshire Police has been supporting the boys' families and is carrying out inquiries alongside the North East Counter Terrorism Unit.
Assistant Chief Constable Mark Milsom said police were "extremely concerned".
He said: "Our priority is for their safe return. Their families are gravely worried about them and want them home.
"Syria is an extremely dangerous place and the public will be aware of the dangers these boys may face.
"The choice of returning home from Syria is often taken away from those that come under the control of Islamic State, leaving their families in the UK devastated and with very few options to secure their safe return."
Kirklees Council chief executive Adrian Lythgo added: "We are deeply concerned for these young people and will continue to work in close partnership with the Counter Terrorism Unit, West Yorkshire Police, our communities and schools to prevent and eradicate any threats and concerns."
The boys were last seen by their families on the day they are believed to have boarded the Thomas Cook flight to Turkey, West Yorkshire Police said.
A spokesman for Thomas Cook Airlines said: "Whilst we cannot confirm the names of any of our passengers who travel with us, we can reassure all our customers that we work closely with all authorities to ensure the safety of our flights in and out of the UK."
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