The top four members of the Senate Armed Services and Foreign Relations committees are warning Turkey it will need to make a choice between the United States and Russia for its defense needs.

In a New York Times op-ed published Tuesday, Armed Services Committee Chairman James Inhofe (R-Okla.) and ranking member Jack Reed (D-R.I.), as well as Foreign Relations Committee Chairman Jim Risch (R-Idaho) and ranking member Bob Menendez (D-N.J.) vowed not to allow Turkey to have both the U.S.-made F-35 aircraft and the Russian-made S-400 missile system.

“By the end of the year, Turkey will have either F-35 advanced fighter aircraft on its soil or a Russian S-400 surface-to-air missile defense system. It will not have both,” the senators wrote.

The United States and NATO ally Turkey have been at odds over Ankara’s plan to buy the Russian defense system.

U.S. officials fear that having the F-35 operate near the S-400 will allow Moscow to gather information on America’s most advanced fighter jet.

The United States has been trying to convince Turkey to buy the U.S.-made Patriot anti-missile system instead of the S-400, but Ankara has said the U.S. terms were not favorable enough.

Last week, the Pentagon announced it was suspending deliveries and activities related to Turkey’s participation in the F-35 program, ramping up the pressure on Ankara to reverse course.

But Turkey is not backing down. On Monday, after a day of meetings with Russian President Vladimir Putin, Turkish President Recep Tayyip Erdo?an again vowed his country would buy the S-400 despite U.S. warnings.

Turkey’s foreign minister also suggested last week that President Trump indicated in a phone call with Erdo?an that Ankara could still get the jets.

“Trump himself admitted on the phone that the U.S. made the mistake not to sell Patriots to Turkey, and he promised Erdo?an that he will take care of this issue,” Foreign Minister Mevlüt Çavu?o?lu said at an event in Washington.

But in their op-ed, the senators vowed that if Erdo?an accepts the S-400, “no F-35s will ever reach Turkish soil.”

“We are committed to taking all necessary legislative action to ensure this is the case,” they wrote.

Inhofe and Reed in particular will be in a position to do something as they write the Senate’s version of the annual defense policy bill. Last year’s National Defense Authorization Act (NDAA) included a provision that blocked delivery of the F-35s until the Pentagon gave Congress a report on U.S.-Turkish relations.

The top Republican on the House Armed Services Committee, Rep. Mac Thornberry (R-Texas), also told reporters Tuesday there is a “strong bipartisan feeling” Turkey should not have the F-35 if it chooses the S-400.

Asked whether the issue should be addressed in the House version of this year’s NDAA, Thornberry said Congress should “see where we are come June” when the House marks up its version.

Removing Turkey from the F-35 program poses complications, however, because key parts of the jet are made there. The Pentagon has said it is looking for alternatives for the Turkish-made parts.

Inhofe, Reed, Risch and Menendez said that while “Turkey is an important partner” in the program, “it is not irreplaceable.”