Britain will live with the consequences of the war in Syria and the rise of Islamic extremism within its own borders for "many years" to come, a top counter-terrorism expert has warned.

Cressida Dick, the Metropolitan police's assistant commissioner and head of specialist operations, spoke after footage appeared online purportedly showing several young British jihadists in Syria in a recruitment video for Islamist terror group Islamic State in Iraq and the Levant (Isis). Reports also surfaced on Sunday that counter-terrorism officers were targeting jihadist recruiters operating within Britain.

"I'm afraid I believe that we will be living with the consequences of Syria – from a terrorist point of view, let alone the world, geopolitical consequences – for many, many, many years to come," Dick told BBC Radio 4's The World This Weekend programme.

In the video, two Britons – now identified in reports as Cardiff schoolfriends Reyaad Khan and Nasser Muthana, both 20, urge more young British Muslims to join their jihad. Further reports on Sunday suggested that Muthana's 17-year-old brother Aseel has become the youngest Briton to join Islamist terror networks involved in the fighting in Syria and Iraq.

According to the report in the Sunday Telegraph, they were part of a wider terrorist network that also included two other men arrested in March and April in the UK after they returned from Syria. That pair, aged 19 and 23, and both also from Cardiff, were held on suspicion of receiving terrorist training and attending a place used for terrorist training, but were later released without charge.

The Muthana brothers' father called on them to come home, saying they were high achievers at school with good career prospects. They must have been "brainwashed", he claimed, and said he believed a network of radical jihadi recruiters must have paid for them to go.

He said he and his wife were "devastated" when they found out from police in November that their eldest son Nasser had joined a terror cell in Syria. In February, police informed them their second son Aseel had obtained a second passport and travelled to Cyprus and was planning to join his brother.

Muthana, 57, told the Sunday Telegraph: "Behind this are Islamic radicals, hiding behind the scenes, influencing the minds of young people. It is not members of the Yemeni community in Cardiff. Someone is persuading them, brainwashing them, helping them travel, arranging tickets."

Nasser was reported to have passed 14 GCSEs, was studying for his A-levels and had been offered places to study medicine at four universities before he became radicalised. His brother Aseel was an A-level student at Fitzalan high school in Cardiff and had dreams of becoming an English teacher.

Muthana, who came to the UK in the 1970s from Yemen, told the newspaper: "I feel sick and devastated my son is caught up in this – they were brought up to love and respect my country Britain. Now I fear they may come back in coffins.

"I'm worried about their safety. But I am also worried about the evil messages Nasser is spreading in this vile video – I am concerned that other boys may follow him there."

And there were further warnings about radicalised Britons returning home. Richard Barrett, a former head of counter-terrorism at MI6, estimated that "possibly up to 300 people have come back to the UK" already, and warned that intelligence services faced an "impossible" task in trying to track them.

He told the Independent on Sunday: "If you imagine what it would cost to really look at 300 people in depth, clearly it would be completely impossible to do that, probably impossible even at a third of that number."

Barrett said police and intelligence resources were stretched in terms of numbers and knowing where the returning jihadists were.

He said: "With this whole business in Syria, although there is no linear projection from foreign fighters to domestic terrorists, it's inevitable that a number will fall into this category."

Barrett has co-authored a report, released this month, which concludes that more than 12,000 foreign fighters have gone to Syria since the war began, and that it is "likely to be an incubator for a new generation of terrorists".

South Wales police said it was increasingly concerned about the number of young Britons travelling to Syria to join the conflict.

In a statement, the force said: "The advice is to avoid all travel to Syria – anyone who does travel is putting themselves in considerable danger. Travelling abroad for the purpose of engaging in terrorist related activity is an offence and we will seek to prosecute anyone engaged in this type of activity.

Police across the UK have made 65 Syria-related arrests over the past 18 months, including 40 in the first three months of this year alone.

On Saturday it emerged that around 500 Britons had travelled to Syria and Iraq – a higher estimate than the 400 claimed by foreign secretary William Hague.

Sir Peter Fahy, who leads on the Prevent counter-terrorism strategy for the Association of Chief Police Officers, said "huge amounts of material" was being taken down from the internet every week as part of the effort to stop people being radicalised.

Another terrorism expert warned that it is "inevitable" that jihadists returning from Syria and Iraq will pose a threat in Britain, saying that hundreds of radicalists may have already returned.

On Saturday thousands of young British Muslim men rallied against radical Islam amid concerns that British jihadists were fighting in Syria.

An estimated 5,000 Muslims gathered in Surrey to pledge loyalty to Britain in light of concerns over the popularity of Isis and the alleged involvement of Britons.

The three-day residential event, organised by the Ahmadiyya Muslim Youth Association (Amya), has brought together 5,000 young British Muslim men from England, Scotland and Wales, to foster bonds of brotherhood and affirm their pride in being British and Muslim.