* Police have arrested 16 people over Syria offences in 2014
* Concerns that people return from war as Islamist radicals
* Britons as young as 17 going to Syria, some are women
LONDON, Jan 25 (Reuters) - Britons travelling to Syria to help the rebels fighting President Bashar al-Assad could be arrested on their return, a senior police chief warned on Saturday, saying they may pose a security risk to the UK.
Peter Fahy, head of the Association of Chief Police Officers, said there was "huge concern" about Britons, including a rising number of youngsters, fighting in Syria and becoming radicalised by hardline Islamists.
British police have already arrested 16 people on suspicion of terrorism offences in Syria this year, some as young as 17, compared to 24 arrests in all of 2013.
Fahy told BBC Radio there was "a real worry about those who may be radicalised, who may have been engaged in terrorist training".
"We stopped quite a number of people because we're very, very clear about what will happen," Fahy said.
Most of the Britons involved in attacks in the UK, including the four suicide bombers who committed the 2005 London bombings that killed 52 people, as well as their co-conspirators, were reported to have received training in camps in countries such as Afghanistan and Pakistan.
A spokesman for London's Metropolitan Police said most of the young Britons going to Syria were men but some were women.
Authorities last week charged the first British women in relation to offences in Syria.
Nawal Msaad, 26, a university student, was arrested at Heathrow Airport and charged with trying to smuggle 20,000 euros ($27,000) to fighters in Syria. She appeared in court but did not enter a plea and was remanded in custody until Jan. 31.
POTENTIALLY A THREAT
Earlier this month, two 17-year-old girls were arrested by police in London and West Yorkshire in northern England heading out of the UK. They were questioned for several days before being released without charge.
"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," the Metropolitan Police spokesman said.
Security assessments estimate up to 500 Britons are in Syria or have been there and returned. This number includes those engaged in aid or humanitarian efforts.
British law was changed last year to make it easier for the government to confiscate the passport from anyone whose "actual or suspected" activities are deemed contrary to the public interest.
Before the changes, passports could only be confiscated if someone was engaged in "demonstrably undesirable" acts, a sanction that was used very infrequently.
Mohammad Ansar, a British Muslim commentator, urged the government to think beyond control orders.
"Rescinding their citizenships is a quick and easy method of keeping them away but it doesn't deal with the underlying problem. There are disaffected Muslim youth in this country and some of them are going overseas," he told the BBC. (Reporting by Belinda Goldsmith; Editing by Robin Pomeroy)