Sweden's security service says up to 150 people have left the country to join IS militants in the Middle East.

Anders Kassman, chief operating officer of the SAPO security service, on Wednesday said the agency has confirmed at least 90 cases of people leaving Sweden to fight in Syria and Iraq. He said 23 of them have been killed fighting for the Islamic State group or similar organizations.

But Kassman said SAPO agrees with Swedish terrorism expert Magnus Ranstorp's estimate that the true number of Swedish foreign fighters is around 150 and increasing quickly.

In August, SAPO reported about 80 confirmed cases.

About 2,000 Europeans are estimated to have joined fighting in Iraq and Syria.