Security officials worry Islamists trained in Syria could target the West
Britons returning to the UK from Syria will be stopped at the border and face arrest, a senior police chief has said.
Sir Peter Fahy, Chief Constable of Greater Manchester Police, told the BBC there was "huge concern" that Britons arriving back after fighting in Syria posed a threat to the UK.
Scotland Yard said its biggest concern was some could return as terrorists.
This month 16 people have been arrested on suspicion of terror offences after travelling between Syria and the UK.
This compares with 24 in the whole of 2013.
A Metropolitan Police spokesman said some of those going to Syria to train in terror camps or fight were as young as 17, and that the majority were young men - although they were aware of some young women also travelling there.
Sir Peter, who leads the Association of Chief Police Officer's "Prevent" strategy on counter-terrorism, told BBC Radio 4's Today programme that those returning from Syria "may well be charged and investigated, but they will be put into our programmes".
"Clearly we've got all sorts of ways of trying to establish that [they have been to Syria]. We have links with intelligence agencies across Europe.
"This is a very difficult situation because Syria is so close. It is very close to tourist destinations, but it is an incredibly dangerous place."
He said those stopped at the border were put on programmes - which saw police work with local agencies such as schools and youth organisations, "essentially to make sure these people haven't been affected and try and make sure they're not a threat to this country".
Sir Peter continued that the main problem was safeguarding the welfare of those going to Syria "who may be driven because of the huge concern over there - some for humanitarian purposes - naively to go out there".
But there was also "a real worry about those who may be radicalised, who may have been engaged in terrorist training", he added.
Militants with suspected links to al-Qaeda have been heading to war-torn Syria from many other countries since fighting broke out in 2011.
Intelligence officials have estimated that the number of UK nationals among them may be in the low hundreds.
Sir Peter Fahy said there was a "real worry about those who may be radicalised, who may have been engaged in terrorist training"
The Centre for the Study of Radicalisation at King's College London says most British jihadists are university-educated Muslims of British Pakistani origin in their 20s.
The Met spokesman said that while the number arrests was relatively low it was "a cause for significant concern, particularly as it is young people who are being enticed to travel to Syria to engage in conflict".
"Our biggest concern is people attending terrorist training camps or fighting in war zones then returning to the UK as terrorists.
"They are potentially a threat to British interests both abroad and at home."
'Boys and girls'
Mohammad Ansar, a broadcaster and social commentator as well as theologian, said the prospect of their return was deeply worrying.
"Once we have these frustrated, often angry and disenfranchised British Muslims going out to Syria to fight, the question is how are we going to reintroduce them to British society?" he told BBC Radio 4's Today programme.
"If they were to come back and if the way they were to express themselves is through violence and the gun then we've got absolutely no control over what they're going to do when they come back into our communities."
On Saturday, members of Syria's government and main opposition group met briefly face-to-face in Geneva as part of a talks process aimed at "saving Syria" - although they did not speak directly.
Syria's civil conflict has claimed well over 100,000 lives since it began in 2011.
The violence has also driven 9.5 million people from their homes, creating a major humanitarian crisis within Syria and for its neighbours.