Islamic State is likely using the bitcoin virtual online currency as a tool to secretly raise funds and settle transactions, a diplomatic source said Tuesday, quoting officials in the U.S.-led coalition against the group.

Members of the coalition worry that the use of bitcoin, which is harder to monitor than bank accounts, would make it even more difficult to choke off the funds and defeat the group, according to the source.

The suspicion about the use of bitcoin reinforces speculation that the social media-savvy group is using cyberspace not only to recruit new members but collect and secure money to fund its activities.