U.S. forces that crossed into Iraq as part of a pull-out from Syria do not have permission to stay and can only be there in transit, the Iraqi military said on Tuesday.
The Iraqi statement contradicts the Pentagon's announcement that all of the nearly 1,000 troops withdrawing from northern Syria are expected to move to western Iraq to continue the campaign against Islamic State militants and "to help defend Iraq".
A senior U.S. defence official later clarified that the situation was still fluid and plans could change.
"All U.S. forces that withdrew from Syria received approval to enter the Kurdistan Region so that they may be transported outside Iraq. There is no permission granted for these forces to stay inside Iraq," the Iraqi military said in a statement.
It is unclear whether the U.S. troops will use Iraq as a base to launch ground raids into Syria and carry out air strikes against Islamic State militants.
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The additional U.S. troops would add to the more than 5,000 American troops already based in the country, training Iraqi forces and helping to ensure that Islamic State militants do not make a comeback.