Sweden's Prime Minister Stefan Lofven
Sweden's Prime Minister Stefan LofvenReuters

Sweden's Prime Minister Stefan Löfven on Thursday admitted his country has been “naive” about the possibility of an Islamic State (ISIS) terror attack, adding Sweden would be stepping up security measures.

"Perhaps it's been hard for us to accept that in the middle of our open, democratic society there are people sympathizing with the ISIL killers," he was quoted by The Local news website as having told journalists at a press conference in Stockholm, using another acronym for ISIS.

Löfven added that among the Swedes travelling to Syria and Iraq to fight for ISIS, there could be people who are also a threat to residents of Sweden.

 "It's completely unacceptable that people can take part in terrorism and return [to Sweden] without being held responsible," he added, according to The Local.

But the Prime Minister also admitted his country was ill-prepared for the growth of Islamist extremism in the Nordics.

"Sweden has been naive, both previous [centre-right] and Social Democrat governments," said Löfven, adding that his government was looking at fresh ways to monitor extremist activities, noting that new technologies such as Skype had made traditional surveillance methods such as phone tapping more or less obsolete.

He suggested that there would be increased secret data and camera surveillance as well as a greater focus on using biometric passport data.

Sweden, as well as Denmark, raised their threat levels earlier this week, in the wake of the Paris attacks which took place last Friday.

"The decision is based on an assessment by the National Center for Terrorist Threat Assessment (NCT) which is responsible for conducting threat assessments for Sweden and Swedish interests abroad," intelligence service Sapo said in a statement.

"One of the reasons for the heightened level is that Sapo has received concrete information and considers that we must act within the framework of our counterterror work," it said.

Sapo revealed last month that around 300 Swedish nationals are believed to have left the country to travel to fight with Islamic extremist groups in Syria and Iraq.

And in July, two Swedish men were charged in Sweden over "terrorist crimes" committed in Syria in 2013, the first in the Scandinavian country to be arrested over such offences.

The suspects, aged 30 and 32, have both denied any wrongdoing when brought before a court.