The U.S. has stopped flying manned air-support missions for rebels in a key part of northern Syria due to Russia’s expansion of air defense systems there, and the Barack Obama administration is scrambling to figure out what to do about it.

This crucial part of the battlefield, known inside the military as Box 4, is where a number of groups have been fighting the Islamic State for control, until recently with overhead support from U.S. fighter jets.

But earlier this month, Moscow deployed an SA-17 advanced air defense system near the area and began “painting” U.S. planes, targeting them with radar in what U.S. officials said was a direct and dangerous provocation. The Pentagon halted all manned flights, although U.S. drones are still flying in the area. Russia then began bombing the rebels the U.S. had been supporting. (U.S. manned airstrikes continue elsewhere in Syria.)

The administration could decide to resume flights in support of the rebels fight Islamic State, but that could risk a deadly incident with the Russian military. For now, the U.S. seems to be acquiescing to Russia’s effort to keep American manned planes out of the sky there and "agree to their rules of the game," the administration official said.