The number of radicalised Islamists in Sweden is now in the thousands, according to the country’s intelligence chief.

Anders Thornberg, head of Sweden’s security service, Säpo, said there had been a rise from 200 in 2010 to a ‘few thousand’ today.

It comes two months after a lorry ploughed into pedestrians in the Swedish capital Stockholm, killing four people.

Uzbek Rakhmat Akilov, 39, who expressed sympathy for so-called Islamic State, has admitted carrying out the attack.

Thornberg called the hike in the number of radicalised Islamists a ‘serious and historic challenge’ but said only a handful of them had the capability or intention of launching an attack.

He said the rise was mainly due to ISIL propaganda, which had succeeded in uniting different groups.

“We had broken groups, we had radicalised communities from North Africa, the Middle East and Somalia, but they were all separated,” Thornberg told TT Press Agency.

Since 2012 around 300 people from Sweden are estimated to have travelled to fight for ISIL in Iraq and Syria.

Nearly one-in-five of this number is aged 19 or under, according to researchers at Swedish Defence University.

The analysis also claims that around 40 percent have since returned to Sweden, while one-in-five have died in Iraq or Syria.