Five youths aged fifteen to nineteen have been arrested in the British capital following a series of city-wide anti-terror raids.

Officers from the Metropolitan Police arrested two males aged 16 and 17 in south London and another two males aged 17 and 19 in west London on January 14, according to the Evening Standard. Another teenager was picked up in east London on February 20 and The Times reports at least one more raid in Lambeth, with police searches continuing as of last night.

Police spokesmen were relatively tight-lipped on the details, but told the press that the five arrests “relate to plans to travel to join a proscribed organisation” – presumed to be the Islamic State, the al-Nusra Front or a similar jihadi terror group.

The suspects are being held under Section 5 of the Terrorism Act 2006, which relates to “preparation of terrorist acts”.

Foreign intelligence experts told The Sunday Times that some 1,600 British nationals had joined the Islamic State as of 2015, with around 500 having returned to UK.

“We believe that even 1,600 is a very conservative estimate because by the time we started picking up on the scale of [the] problem, a lot of the people had slipped through the gates,” said one source.

Even the somewhat lower estimates of the British authorities indicate that jihadist groups were more successful in recruiting British nationals than the Army Reserve in 2014. Of the 400 extremists thought to have returned to the UK, only fourteen were known to have been identified and imprisoned as of October 2016.

Reports indicate that hundreds of young British Muslims are still eager to travel to the Islamic State – despite a series of damaging reverses inflicted on the terror group by Russia and a little-reported Turkish land invasion of Syria, aimed nominally at ISIS but alleged to be principally concerned with eliminating the Kurdish forces previously considered the West’s most effective counter-jihadi ground assets.