The US senators sent a letter to the Pentagon head Mark Esper to speed up Turkey's withdrawal from F-35 program, Al Monitor reported.

The letter was signed by Senators James Lankford, Jeanne Shaheen, Thom Tillis, and Chris Van Hollen.

“Unfortunately, Turkish manufacturers are still producing and delivering key components of the aircraft despite the statutory prohibition on such participation in manufacturing line of the program,” 

“We believe more urgency is needed and hope you will accelerate the process to ensure a more prompt removal.”

The US officially excluded Turkey from the F-35 fighter jets co-production program amid Turkey’s purchase of the Russian S-400 missile defense systems.