A Pentagon spokesman says the U.S. has told Syria that its air force must not fly in areas where American military advisers are working with Kurdish and Arab forces fighting the Islamic State group. But he said this does not amount to a "no fly zone."

Reporters on Monday pushed press secretary Peter Cook to explain the distinction.

Cook says the Syrians have been warned repeatedly that the U.S. will defend its forces on the ground, and that their warplanes should not fly in areas near U.S. troops. Twice last week, Syria aircraft flew in a part of northeastern Syria where U.S. forces are advising local anti-IS forces, prompting U.S. warnings.

Cook says the warnings were communicated to the Syrians through the Russian military.