Four U.S. senators on Thursday introduced a bipartisan bill to prohibit the transfer of F-35 fighter aircraft to Turkey until the U.S. government certifies that Ankara will not take delivery of a Russian S-400 air defence system, a statement on the move said.

U.S. Senators Jeanne Shaheen (D-NH), James Lankford (R-OK), Thom Tillis (R-NC), and Chris Van Hollen (D-MD) today introduced a bill to prohibit the transfer of F-35 Joint Strike Fighter aircraft to Turkey until our government certifies that Turkey will not accept delivery of Russia’s S-400 air defence system. Under the US-led, multinational Joint Strike Fighter program, Turkey is expected to accept delivery of the F-35 as early as next fall.

“Turkey is an important NATO ally and willing partner in addressing a number of US national security priorities,” said Senator James Lankford in a press release.

Lankford, however, shared concerns about Turkey's plan to obtaining the S-400 system, saying that Russia “seeks to undermine NATO and US interests at every turn.”

The bill defines the “transfer” of the F-35s as being relocated outside the continental U.S. and does not apply to fighter jets being operated by American forces.

Two F-35s have already been delivered to Turkey, and are currently at Luke Air Force Base in Arizona, where Turkish pilots are training with the aircraft. These jets were scheduled to be transferred in November. Two new jets are recently produced and were ought to be delivered to Turkish Air Force at the beginning of March.