The U.S. State Department has notified Congress of a proposal to sell the Patriot air and missile defense system to Turkey, a move that signals a key breakthrough with a NATO ally that had been inching closer to Russia.

The potential $3.5 billion deal for the system, which is manufactured by Raytheon Co. and Lockheed Martin Corp., would help bolster Turkey as it confronts instability in the region, according to a statement by the Defense Security Cooperation Agency posted on Tuesday.