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.)

Inside the top levels of the administration, officials are debating what to do next. The issue is serious enough that Secretary of State John Kerry raised it with Russian President Vladimir Putin when they met on Tuesday, and Joint Chiefs of Staff Chairman General John Dunford has discussed it with his Russian counterpart as well, a spokesman for U.S. Air Force Central Command told us.

“The increasing number of Russian-supplied advanced air defense systems in Syria, including SA-17s, is another example that Russia and the regime seek to complicate the global counter-Daesh coalition’s air campaign,” said Major Tim Smith, using another term for the Islamic State.