The families of two "ordinary Yorkshire lads" who are believed to have travelled to Syria have issued pictures of the teenagers.
Hassan Munshi and Talha Asmal, both 17, are thought to have fled to the war-torn country after travelling to Turkey on March 31.
Earlier this week, police said t he boys from Dewsbury were last seen by their families on the day they are believed to have flown from Manchester to Dalaman.
West Yorkshire Police said it has been supporting the missing teenagers' families and carrying out extensive inquiries alongside the North East counter-terrorism unit.
A statement released on behalf of the two families on Tuesday said they are devastated at what has happened.
It said: " Our number one priority is to get Hassan and Talha back home with their families and we implore anyone who may have any information whatsoever to get in contact with the police.
"Naturally, we are in a state of profound shock and are trying to come to terms with the predicament we find ourselves in and we hope and pray that no other family finds itself in our situation.
"These were just two ordinary Yorkshire lads who enjoyed the things that all young people enjoy at their age - both Hassan and Talha had a promising future, as an apprentice and an A-level student respectively, and we are praying they will be back with us soon and are able to realise that future."
The families have urged parents to be e xtra vigilant, saying it is "near impossible" to know if children have been groomed and brainwashed.
Hassan is believed to be related to Hammaad Munshi, who was arrested by counter-terrorism police in 2006 at the age of 15 and later convicted over his role in a plot to murder non-Muslims, The Times newspaper reported.
The Metropolitan Police believe around 600 Britons have travelled to Syria and Iraq since the conflict began, while around half are believed to have returned to the UK.
Nine Britons - including the son of a Labour councillor - were arrested in Turkey last week on suspicion of trying to cross the border into the neighbouring country.
Waheed Ahmed was one of five adults and four children being held by Turkish authorities after they were stopped in Hatay province last Wednesday.
Meanwhile, teenage girls Shamima Begum, Amira Abase and Kadiza Sultana are believed to be inside Syria after flying to Turkey in February.
It is understood the Bethnal Green Academy pupils from east London were following another 15-year-old girl who travelled to Syria in December.