U.S. Secretary of State Mike Pompeo said on Thursday the United States was continuing to evaluate how it would respond to Turkey’s purchase of Russian S-400 missile defence systems.

“We continue to evaluate how to apply sanctions in order to achieve our end objective,” Pompeo said under questioning from Democratic Senator Bob Menendez at a Senate Foreign Relations Committee hearing.

Turkey’s decision to acquire the Russian-made S-400 surface-to-air missile systems has caused a sharply negative reaction from the United States and NATO as a whole. The United States insists that Turkey must give up the Russian air defense systems.

As Turkey has said that it will not give up the S-400 systems, Washington has excluded Ankara from the US program of developing the fifth-generation F-35 fighter-bomber.