The U.S. has expressly warned Syria to keep its warplanes away from a wide area of its territory where U.S. special operations forces are advising so-called partner forces in their battles against the Islamic State. Yet the Pentagon insists it has not imposed a "no-fly zone."

"It's not a no-fly zone," said Pentagon Press Secretary Peter Cook, "but the Syrian regime would be wise to avoid areas where coalition forces have been operating, and we will continue to defend them and if need be we will send aircraft again to defend our forces."

Cook said the air exclusion zone is to protect U.S. forces, and is consistent with long-standing policy that the U.S. would use air power if necessary to not only defend American troops, but also the forces they are backing against the Islamic State. The zone is located over the northeast corner of Syria, which includes Manbij and Hasakah, where U.S. troops are serving as advisers to local fighters.

Pressed to explain how a zone where the U.S. says Syrian planes can't fly is not the same as a "no-fly zone," Cook said, "You can label it what you want. It remains the same warning we've had in effect since we started our operations in Syria, we are going to defend our people on the ground, and do what we need to do to protect them."

The Pentagon, along with various U.S. commanders, have argued that establishing a no-fly zone to protect Syrian civilians on the ground from air attack from the regime of Bashar Assad would not be practical.

But when U.S. special operations forces who were working with Kurdish fighters were dangerously close to a location bombed by Syrian planes Thursday, the U.S. was quick to scramble fighter jets from Turkey and begin patrolling over the area.

On the following days, coalition fighters chased two Syrian Su-24 warplanes away from the area, effectively enforcing the ban on flights that the Pentagon argues is not a no-fly zone.

In answer to a reporter who asked if the U.S. military is prepared to shoot down a Syrian or Russian aircraft that violated the ban, Cook said "If they threatened U.S. forces we always have the right to defend our forces."