(Bloomberg) -- 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.

“This proposed sale will contribute to the foreign policy and national security of the United States by improving the security of a key NATO Ally on the front lines of the fight against terrorism,” according to the statement.

Tensions between the U.S. and Turkey nations have been heightened in the last few years as the government of President Tayyip Erdogan turned more authoritarian and the two allies disagreed over U.S. support for Kurdish militias in Syria. In recent days, Erdogan threatened to send his forces in Syria toward Kurdish-held areas where U.S. troops are operating, raising the prospect of a battlefield confrontation between NATO’s two biggest armed forces.

Trump administration officials seeking better relations were stymied by a disagreement that threatened to turn into a crisis: Turkey’s intention to purchase an advanced Russian anti-aircraft missile system, the S-400 Triumph.

Speaking to reporters in Washington last month, Turkish Foreign Minister Mevlut Cavusoglu said his country had wanted to buy Patriot missiles but was never able to get a commitment from Washington. He said there was no going back on the S-400 deal with Moscow but left the door open to buying U.S. hardware in the future.

“It’s a big deal, because only a few months ago, analysts were predicting all doom and the end of nearly seven-decades-old U.S.-Turkey relationship,” said Soner Captagay, the director of the Turkish research program at the Washington Institute for Near East Policy. With the Trump administration “agreeing to share some of its most sensitive technology to Ankara, this suggests a new phase in U.S.-Turkish ties, one driven by a new rapport between Presidents Erdogan and Trump, and aided by bureaucracies in both countries determined not to let the US-Turkish relationship collapse,” Capagay said.

The proposed deal, which must be approved by Congress, includes up to 140 Patriot missiles, radar and ground control stations. Congress has 15 days to review the proposed sale under streamlined procedures the State Department has in place for NATO allies.

The complication over the S-400 was not the only obstacle to better U.S.-Turkish ties. Turkey continues to detain a NASA scientist and Turkish employees of the State Department after freeing a detained American pastor earlier this year. Erdogan has been frustrated by the conviction in a U.S. court of a Turkish banker on sanctions-violation charges. And the Ankara government demands that the Turkish cleric Fethullah Gulen, a former Erdogan ally who has been living in the U.S., be extradited.

To contact the reporters on this story: Tony Capaccio in Washington at acapaccio@bloomberg.net;Nick Wadhams in Washington at nwadhams@bloomberg.net

To contact the editors responsible for this story: Bill Faries at wfaries@bloomberg.net, John Harney

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