Iraqi Kurdish forces are a key US partner in the war against IS, which overran large parts of Iraq last year.
Carter's visit to Arbil, the capital of Iraq's autonomous Kurdish region, followed a trip to Baghdad the day before an AFP journalist said.
There, he met Prime Minister Haider al-Abadi, and the two discussed "improving cooperation between the two countries in the fields of arming and training," a statement from the premier's office said.
Carter and his delegation also met US partners in the international anti-IS coalition.
President Barack Obama said on Monday the US and its allies were hitting IS "harder than ever" and warned the extremists' leaders: "You are next.