Several members of the US Congress were reportedly in Turkey on Monday, where they encouraged officials to give up their bid to acquire the Russian-made S-400 missile system.

Ankara’s deal to purchase the advanced weapons has emerged as one of many flashpoints in its relationship with Washington, spurring US lawmakers to pass legislation delaying the delivery of F-35 fighter jets to Turkey.

While lawmakers voiced concern that Russia could collect intelligence on F-35 technology through the S-400 systems, the Pentagon has been determined to keep Ankara in the F-35 Joint Strike Fighter program. Because of the JSF’s consortium model of production and maintenance, cutting Turkey out would cause production delays of up to two years, the defense department says.

Congress is currently reviewing a report from the Pentagon detailing the costs that would be incurred should Turkey be kicked out of the program.

According to Hurriyet Daily News, the three-member congressional delegation traveling to Turkey this week suggested that the F-35s would only be delivered if Ankara agreed not to purchase the Russian missile systems.

In response, Turkish officials urged cooperation in fighting the Gulen movement. The administration of Turkish President Recep Tayyip Erdogan has repeatedly called for Washington to extradite the movement’s leader, Fethullah Gulen, who is in exile in the United States for allegedly orchestrating a failed coup attempt in 2016.