The State Department continues to hold the door open to Turkey to rejoin the F-35 Joint Strike Fighter program if it opts to procure a compatible integrated air defense system instead of the Russian-made S-400.

“There is certain room to course-correct” since the Pentagon announced last spring that it planned to eliminate Ankara’s role as a development partner of the F-35, replacing local suppliers and holding F-35A aircraft destined for Turkey at Luke Air Force Base, Arizona, said R. Clarke Cooper, assistant secretary of state in the bureau of political-military affairs, at a Feb. 14 media roundtable at the annual Munich Security Conference.

“There is room to reconcile, but the longer this is protracted, the greater they’re at risk of cleaving or extracting further parts of the relationship,” Cooper said, according to a transcript provided by the State Department. “That said, we are working very hard to maintain the bilateral relationship on a military-to-military basis.”

The White House officially announced Turkey’s removal from the F-35 program last July, meaning Ankara would no longer receive 100 F-35As it had committed to purchasing, due to the country’s decision to procure the S-400, a Russian-made air-to-air defense system that officials and experts have deemed “incompatible” with the NATO-flown F-35. Turkey is a member nation of NATO.

Pentagon leaders have said the Defense Department would spend up to $600 million searching for alternate sources for over 900 Turkish-made parts. Undersecretary of Defense for Acquisition and Sustainment Ellen Lord told reporters in January at a Defense Writer’s Group breakfast in Washington, D.C., that the “majority” of the supply chain will be out of Turkey by March 2020, but some contracts may continue through the end of the calendar year.

“We continue to manage the program to minimize impact to production,” she said. Lockheed Martin [LMT], the prime contractor, and Pratt & Whitney [UTX], the engine supplier, have contracts that could affect a handful of parts and run through the end of 2020, she added.

The U.S. government has continued to lobby Turkey to buy Raytheon [RTN]-built Patriot batteries instead of the S-400. The State Department approved a $3.5 billion potential sale to Ankara in December 2018. Lawmakers included in the fiscal year 2020 National Defense Authorization Act a bill to block F-35 sales to Turkey so long as the S-400s are in country.

Turkish media outlet Ahval reported Jan. 30 that Turkish President Recep Tayyip Erdoğan told local reporters that the United States will either deliver the Joint Strike Fighters as contracted, or return the payments already made on the aircraft. If not, “We will find and purchase them from another place, or we build them ourselves,” he said.

Erdoğan added that “The S-400 issue is over,” per the report. “It is out of the question for us to turn back from this deal.”

Cooper said at the Munich Security Conference that Turkey is at risk of “further isolation” and increased sanctions if it continues to acquire the S-400 or opts for a fighter aircraft such as the Sukhoi Su-35, as it is reportedly also interested in procuring.

“The United States has a number of sanction considerations for them, and it’s important for our partners in Ankara to appreciate is that just because sanctions haven’t been issued to date does not mean they will not be issued,” Cooper said.