Four senators introduced a bill March 28 that would prohibit the foreign military sales of Lockheed Martin’s [LMT] F-35 Joint Strike Fighter to Turkey so long as the NATO member state continues the procurement of Russia’s S-400 missile system.

Sens. Jeanne Shaheen (D-N.H.), James Lankford (R-Okla.), Chris Van Hollen (D-Md.) and Thom Tillis (R-N.C.) said in a Thursday release announcing the new bill that while Turkey remains a valuable NATO ally, they remain concerned about the implications of placing a Russian-made weapon system onto the F-35, the U.S. military’s most sophisticated fighter jet used by partner nations around the globe.

The new act is referred to as the “Protecting NATO Skies Act of 2019” in the bill language, and it requires a written certification from the U.S. government that Turkey has withdrawn its decision to procure the S-400 before an F-35 can be transferred to the country.

An F-35 Lightning II streaks across the sky while doing maneuvers to the Eglin Air Force Base runway. The 33rd Fighter Wing-owned aircraft is a fifth-generation fighter and used to train pilots and maintainers. (U.S. Air Force photo/Samuel King Jr.)

“The prospect of Russia having access to U.S. aircraft and technology in a NATO country, Turkey, is a serious national and global security risk,” said Shaheen, who sits on the Senate Armed Services Committee and Senate Foreign Relations Committee. “This bill makes it clear that NATO’s integrity, interoperability and security is a top foreign policy concern across all branches of the U.S. government.”

“This bipartisan legislation would draw a hard line in the sand and protect vital national security interests, and the Senate should consider it without delay,” Van Hollen said.

Tillis, a SASC member, urged Turkey to “cease this deal immediately so we can deliver the F-35 as originally planned and continue to work towards advancing the common interests of our countries.”

Lankford said the bill “sends a clear message to the Turkish government that it cannot have sensitive, state-of-the-art American military technology and Russian military technology.”

Turkish President Recep Tayyip Erdogan asserted March 24 that his country would not go back on the commitment to procure the S-400, multiple outlets have reported.

A Lockheed Martin spokesperson referred Defense Daily to the Department of Defense for comment Thursday.

Earlier this month, outgoing U.S. European Command Commander Army Gen. Curtis Scapparotti told the Senate Armed Services Committee during a hearing that his “best military advice” would be to curb sales of the F-35 to Turkey should they continue with the sale of the S-400.

That being said, Acting Defense Secretary Patrick Shanahan told the House Armed Services Committee (HASC) this Tuesday that he supported Ankara remaining in the F-35 program because “we need Turkey to buy Patriot,” referring to the Lockheed Martin-Raytheon [RTN] air and missile defense system.

The NATO ally, which hosts the U.S. Air Force’s Incirlik Air Force Base, has a program of record for 100 F-35A aircraft with Lockheed Martin via foreign military sales. A number of Turkish pilots have been training at Luke Air Force Base, Arizona. reported Monday that two new F-35A aircraft meant for Turkey were expected at Luke AFB next month.

Shaheen, Lankford and Tillis introduced a similar bill last year to restrict the sale of fighter aircraft to Turkey. A version of the bill was included as an amendment in the fiscal year 2019 National Defense Authorization Act, which directed the Secretary of Defense to submit a plan to Congress to remove Turkey from the F-35 program.