STOCKHOLM (AFP) - Swedish police detained four people on Tuesday on suspicion of having used six companies to help fund the Islamic State in Iraq and Syria (ISIS) group, the Dagens Nyheter daily reported.

Police said a "large raid" took place in Stockholm as part of a probe into money laundering and undeclared work but declined to comment further.

Sources close to the investigation told Dagens Nyheter the suspects had ties to Syria and ISIS.

The paper said Tuesday's arrests targeted three men and a woman aged between 31 and 50.

Swedish government agencies estimate that up to 10 million Swedish kronor (S$1.6 million) are sent to violent extremist groups every year.

"A few million may sound like not a lot of money, but remember that an act of terror rarely costs more than 100,000 kronor to execute," Jonas Clausen Mork at the Swedish Defence Research Agency told Dagens Nyheter.

In January, the Swedish intelligence service said 130 Swedes had joined Islamic extremist groups in Iraq and Syria since 2012.