The Islamic State (IS) group's financing is likely to shift from a "centralised" system in Iraq and Syria to a much more fragmented one following the death of its leader, a US official said Wednesday.

"They still have access to millions of dollars," Marshall Billingslea, US Treasury assistant secretary for terrorist financing, told reporters after attending a Luxembourg meeting focused on countering the IS.

He said that the group's financial operations are expected to move away "from a centralised model in Iraq and Syria into a much more regionalised approach" as it adapts to cope with the death of leader Abu Bakr al-Baghdadi. Baghdadi died on October 26 during a US special forces raid on his hideout in northwestern Syria, where he was holed up after the defeat of his so-called caliphate that has once stretched across northern Syria and Iraq.