Carter, who yesterday visited a Turkish base that has become a key hub for air raids against the jihadists, arrived in the Iraqi capital today.
He was due to leave Iraq the same day after meeting Iraqi leaders and partners in the US-led anti-IS coalition, an official said on condition of anonymity.
US aircraft carry out daily air strikes against IS targets, most of them in the Iraqi part of the jihadists' self-proclaimed "caliphate", which also covers regions in Syria.
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.