"A story like this always has a history," he said in an interview with public broadcaster ZDF. "Who is financing these troops? Hint: Qatar."
Vice Chancellor and Economy Minister Sigmar Gabriel this week urged a "debate" about who has been and is financing the IS, but without naming any countries.
Germany is expected to decide in coming days whether to deliver non-lethal military aid or possibly arms to Iraqi Kurds fighting the jihadists.
It is a controversial issue for Germany which, burdened by its history of aggression in two world wars, has often been reluctant to send troops into foreign conflicts and which as a rule does not send weapons into war zones.