David Cameron says the IS threat to teenagers has changed
The UK must "get to grips" with the changing face of youth radicalisation, David Cameron has told the BBC.
The Prime Minister said the programmes that focus on de-radicalising boys must turn their attention to girls too.
He said: "In the past the danger has been more about boys but we can now see that is changing."
The struggle to prevent young people from joining the Islamic State could go on for "not just years, but possibly decades", he warned.
He told the BBC's Asian network reporter, Poonam Taneja, that all parents had responsibilities towards their children. But he added: "I think my message to everyone is that we are all in this struggle against radicalisation together.
"We can't entirely say it's a matter for the police or the border force, everyone has to be involved in spotting signs of radicalisation and combating those signs".
He said that programmes focusing on de-radicalising must look at everyone: "Clearly that means girls as well as boys".
"It is a really big issue for our whole country to get to grips with and work out how we put a stop to it," he added.
'Very frightening prospect'
Mr Cameron's comments came as the families of three UK schoolgirls missing in Syria appeal for them to come home and after three young British men were stopped from travelling to Syria from Turkey over the weekend.
Mr Cameron said it was a "very frightening prospect" that the young girls would leave Britain - "where you can have real opportunities and real freedoms" - to travel to Syria to be part of an "appalling death cult".
Young people who attempt to travel to Syria to join Islamic State are "deeply misguided", Mr Cameron said in a separate interview.
He told BuzzFeed that anyone attempting such a journey should be returned to the UK so they could "get this radical nonsense out of their heads".
The three young men, two aged 17 and one 19, from north-west London, were arrested after being returned to the UK and released on bail.
They are the latest in a number of young people from the UK who have attempted, and in some cases succeeded, in reaching the terror group which has a presence in Syria.
Mr Cameron said it was right that the three teenage boys had been arrested, adding that he now wanted to see the three girls brought back also.
Speaking about young British people who wanted to join IS, he said: "I think they are deeply misguided and they are potentially going to join a criminal organisation which could make them part of a criminal or terrorist conspiracy.
"We want to get them back and try to get this radical nonsense out of their heads.
"That people in an outstanding school can opt to go and join a death cult in Syria that believes in throwing gay people off buildings and cutting people's heads off in the desert, is deeply depressing and we should be really worried about this as a country."
The three Britons stopped in Turkey were apprehended after UK police alerted Turkish officials after a tip-off from the younger teenagers' parents, the Times has said.
The two 17-year-olds were stopped at Istanbul's Sabiha Gokcen airport by Turkish authorities acting on intelligence provided by British police.
However, the 19-year-old man was only detained after being questioned by Turkish police, the official said. He also was arrested at the airport.
BBC correspondent Andy Moore said the development came after "recriminations" between UK police and Turkish officials following the disappearance of the three girls from east London.
Their disappearance led to criticisms from Turkey's deputy prime minister, who said Turkish officials had not been given enough warning about their disappearance.
Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16 took flights to Istanbul last month, from where it is feared they travelled to join IS militants in Syria.
A senior Turkish government official told the BBC that Turkish security agencies have drawn up a "no-entry" list of 12,500 people, with some volunteers being as young as 14.
They have also deported more than 1,100 people suspected of wanting to join Islamic State (IS).
Speaking on condition of anonymity, he complained that Western intelligence agencies are not doing enough to prevent the would-be fighters leaving their countries of origin.
Dominic Casciani, BBC home affairs correspondent
Last year there were 327 terrorism-related arrests, more than half of them related to Syria, leading to 64 charges and prosecutions.
These numbers - and regular reports that some young people are still trying to get to Syria - are evidence enough that the problem isn't going to be solved by arrests alone.
The police will arrest and charge if there is clear evidence that someone has committed a terrorist offence.
But they will also look at other tactics to disrupt someone's intentions. Their counter-terrorism toolbox now includes beefed-up powers to stop people at ports, seize passports and place the most dangerous suspects under near-constant monitoring.
But most importantly, the recently passed Counter-Terrorism and Security Act places a legal duty on public authorities to start doing more to combat extremism.
The police will continue to make arrests - but the focus is increasingly moving to the really difficult business of preventing someone being radicalised in the first place.
About 600 Britons are believed to have travelled to Syria and Iraq since the conflict began, according to Metropolitan Police Assistant Commissioner Mark Rowley.
The Home Office estimates around half of those have returned.
The BBC understands more than 500 Britons are believed to have travelled to the region to join militant organisations, while 100 Western volunteers, including some from the UK, went to fight with the Kurdish forces against IS.
Jamshed Javeed, a teacher from Bolton who admitted Syria-related terror offences, was jailed for six years earlier this month.
The news comes as the National Police Counter Terrorism Network and partners have rolled out an advertising campaign designed to reach out to families, to prevent young people travelling to Syria.
In the last year 22 women and girls have been reported missing by families who feared they had travelled to Syria.
Deputy Assistant Commissioner Helen Ball, counter-terrorism co-ordinator, said police were increasingly concerned about the numbers of young women who have travelled or are intending to travel to Syria.