Stockholm - Sweden is home to some 2,000 Islamist extremists, the nation's intelligence chief said on Monday, a nearly tenfold increase in less than a decade.
Anders Thornberg, head of domestic spy service Sapo, attributed the rise primarily to the sophisticated propaganda machine of the Islamic State group.
Although "few extremists" have "the will and ability" to carry out attacks, they must be found and closely followed, Thornberg said.
"It's important that everyone in Sweden takes responsibility to end this trend... before we see an attack or a violent act," Thornberg told news agency TT in an interview published on Monday.
Sweden has been on the edge ever since an Uzbek national, who had shown sympathies for jihadist groups including IS, used a stolen truck to mow down pedestrians on a busy shopping street on April 7, killing five people and injuring 15.
Europe has seen a string of attacks in recent years including large-scale assaults in Paris Brussels and Berlin.
Thornberg said that out of around 3,000 violent extremists currently in Sweden, 2,000 have Islamist motives. The remaining extremists originate from far-right and extreme-left movements.
A 2010 Sapo report estimated the number of violent Islamist extremists in the Scandinavian country at 200.
Sapo has previously said that about 300 people from Sweden are known to have travelled to Syria and Iraq to join IS since 2012. Around 140 have returned to Sweden and about 50 are said to have died abroad.
Jihadists or sympathisers from Sweden have been linked to several terrorist attacks in recent years, as a Swedish national, Osama Krayem, has been charged with terrorist murders over the 2016 Brussels metro bombing.