British police launched a campaign on Thursday urging Muslim mothers to inform the authorities if they fear their children may go to fight in Syria, although critics warned that families were often the last to know.
Counter-terrorism officials warned of the risks young people face by travelling to the conflict-ravaged country, from kidnapping and death to criminal prosecution in Britain if they became involved in terrorism.
They urged people to channel their desire to help with the humanitarian crisis by supporting reputable charities working in Syria, but to refrain from going there themselves.
However, critics questioned whether families would inform on their own children in the knowledge that they might go to jail, even if they knew what they were planning.
"All the evidence indicates that the families themselves are the last to know. They are also most unlikely to tell the police," said Keith Vaz, chairman of parliament's home affairs committee.
Hundreds of Westerners have joined the groups fighting President Bashar al-Assad's regime in Syria over the past three years, many travelling there by road from Turkey.
On Wednesday, France unveiled new plans designed to prevent its citizens from joining the fighting, including a number for concerned relatives to call, and a mechanism for parents to prevent under-18s from leaving the country.
Under the plans -- announced after a group of French hostages freed after being held in Syria for 10 months revealed that some of their captors were French-speakers -- the government will also be able to seize the passports of known jihadists.
Aside from the risks in Syria, there are fears returning fighters could use their training to commit attacks at home.
The new British campaign was prompted by a jump in Syria-related arrests, from 25 last year to 40 in the first three months of this year alone.
"We are increasingly concerned about the numbers of young people who have or are intending to travel to Syria to join the conflict," said Helen Ball, senior national coordinator for counter-terrorism at London's Metropolitan Police.
"We want to ensure that people, particularly women, who are concerned about their loved ones are given enough information about what they can do to prevent this from happening."
She added: "This is not about criminalising people, it is about preventing tragedies."
Police officers and community groups held meetings with women across the country on Thursday, encouraging them to work with the authorities.
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"We know that mothers are key agents of change -- they are the first ones who notice any signs, any change of behaviour," said Sajda Mughal, who runs London women's community group the JAN Trust.
Police say any cases raised with them will be dealt with in confidence, although Ball said there were some situations where police involvement was inevitable.
"Fighting, full stop, and training to fight are likely to mean a police investigation," she said.
- Ending up dead -
Ball said the numbers of Britons travelling to Syria were in the low to mid-hundreds, although other estimates put the figure at 400, with 20 Britons having been killed.
Last week, a father from Brighton on England's south coast, Abubaker Deghayes, revealed that one of his sons had been killed in Syria and two of his other sons were fighting there too.
Abdullah Deghayes, 18, died in battle earlier this month and his 20-year-old brother Amer was shot in the stomach. Amer and his 16-year-old brother Jafar remain in Syria.
The teenagers' aunt, Amina Deghayes, said separately her family had received no advance warning of their intentions and questioned whether the police campaign would be effective.
"If the steps are to speak to the guys before they leave, I think people already have -- they do not need the government to tell them that," she said.
"In the case of my nephew, he ran away. At what point would we speak to him?"
She added that since her nephews had been in Syria, their relatives had failed to persuade them to come home -- and the threat of prison in Britain did not help.
"What can they (the young men) do? Even if they do change their minds, they are caught between a rock and a hard place," she said.
Vaz said the issue needed to be addressed at a peer group level, with a blunt message: "Young people need to be persuaded that if they go to Syria they may end up dead."