STOCKHOLM (Reuters) - The number of Islamist militants in Sweden has soared to thousands in recent years but only a few pose a security threat to society, the head of the country’s security services said on Friday.

Sweden is still in shock after five people were killed and 15 injured when a hijacked truck ploughed into a crowd on a busy shopping street and crashed into a Stockholm department store on April 7.

Police are holding an Uzbek man who has admitted to driving the vehicle.

Anders Thornberg, the head of the Swedish Security Service (Sapo), said only a handful of militants had the desire and capacity to carry out attacks. He blamed propaganda by Islamic State for the problem.

“We have never seen anything like this before,” Thornberg told national news agency TT.

He said the numbers included those who merely sympathise with violent militants to those who spread the message, recruit and collect funds.

“The growth in extremism is a challenge of historical proportions,” he said.

The biggest concentrations of militants are in Stockholm, Gothenburg, Malmo and Orebro, according to Sapo.

The government has tightened laws and promised more funding for police and security services. It is planning further measures, including increased CCTV surveillance.

But Thornberg said the security services faced a new environment where attacks no longer needed months of planning and preparation.

“Today, if you decide to act, you maybe buy two knives or hire a truck and drive into a crowd,” he said.