Iraqi forces backed by a U.S.-led international coalition last year captured all the territory that fell under Islamic State control in 2014 and 2015, including the northern city of Mosul, which served as the militants' de facto capital.
"The American forces have begun reducing their numbers as victory has been achieved over Daesh," the spokesman told Reuters. "Coordination continues, to maintain (U.S.) assistance to the Iraqi forces in accordance with their requirements."
The United States had more than 5,500 troops in Iraq at the height of the battle of Mosul in July 2017, making up about half the total force deployed by the coalition in the country.
A coalition spokesman declined to confirm or deny that the drawdown had begun. "It is our intent to publish a release whenever we have forces moving out of the theater," the spokesman, U.S. Army Col. Ryan Dillon, told Reuters.
"Continued coalition presence will be conditions-based, proportional to the need and in coordination with the government of Iraq," he said.