Hillary Clinton has proposed establishing a no-fly zone in Syria, but Director of National Intelligence James Clapper said Tuesday that such a move could lead Russia to shoot down U.S. planes.

“I wouldn’t put it past them to shoot down an American aircraft if they felt that was threatening to their forces on the ground,” Clapper said during a talk at the Council of Foreign Relations.

Russia recently deployed mobile S-400 and S-300 missile batteries to Western Syria, which are capable of shooting down aircraft and cruise missiles. Clapper signaled that they posed a threat to American planes should the U.S. try to institute a no-fly zone in Syria.

“The system they have there is very advanced, very capable,” said Clapper, “and I don’t think they’d do it – deploy it – if they didn’t have some intention to use it.”

In the past, Clinton has called for “safe zones” and “no-fly zones” in Syria. During the third presidential debate last week, moderator Chris Wallace asked Clinton whether she would fire on Russian aircraft that were violating a U.S.-imposed no-fly zone. She did not answer the question, saying only that a no-fly zone “would take a lot of negotiation,” and that she thinks she could “strike a deal” where Russian planes would avoid no-fly zones.

General Joseph Dunford, the Chairman of the Joint Chiefs of Staff, told a Senate panel last month that for the U.S. to “control all of the airspace in Syria, it would require us to go to war, against Syria and Russia.”

President Obama dismissed calls for a no-fly zone last year, calling the idea “half-baked.”