IS, which is also known as Daesh, has lost an estimated 90 per cent of its territory in Iraq. At one time, the group held around half of Syria but today controls just 15 per cent, according to estimates.
"As we have success against Daesh on the ground in Iraq and Syria, they are moving funds out of Iraq and Syria," Julian King told the civil liberties committee in the European Parliament.
Last month, a UN report said that IS was continuing to send remittances abroad -- often small sums, making them difficult to detect -- as part of a bid to step up its international efforts "as demonstrated by the higher pace of attacks in Europe."
The report said funding sources were still based on oil profits and the imposition of taxes on local populations in the areas under its control.
In the last two or three years, EU member states have been hit by an increasing number of attacks claimed by IS which have taken place in Spain, Britain, France, Belgium, and Germany.
King said the EU is also stepping up efforts to "respond to the unprecedented scale and speed of radicalisation that we now face in our societies," including online and offline.