US forces crossed into Iraq from Syria from the Suhaila border crossing in the northern province of Dohuk.
Reuters footage showed armored vehicles carrying troops to Iraq as part of the US withdrawal from Syria. The agency's photographer said he had seen more than 100 armored vehicles crossing.
An Iraqi Kurdish security source told Reuters that US forces had crossed into Iraq's semi-autonomous Kurdistan region. He said about 30 trailers and Hummer vehicles carrying equipment had crossed, as well as vehicles carrying soldiers.
A second security source in the northern city of Mosul said US troops had crossed into Iraq from the Suheila crossing.
US Defense Secretary Mark Esper said on Saturday that all US troops withdrawing from northern Syria - about 1,000 - are expected to move to western Iraq to continue the campaign against ISIS and "help defend Iraq."
The withdrawing US troops will join more than 5,000 US troops in Iraq. Esber said he had spoken to his Iraqi counterpart about the additional troops.
Turkey and the United States agreed last Thursday to suspend the Turkish "peace spring" operation in northeast Syria for five days to allow time for Kurdish fighters to withdraw from a "safe area" that Ankara aims to establish near its border with Syria.