The United States says it has no plans for the time being to accept any additional Turkish pilots to train on F-35 fighter jets as part of a planned program between the two countries.

Two unnamed US officials told Reuters on Friday that Washington has decided not to accept any new Turkish pilots.

They, however, added that the decision could be reversed if Ankara ditches its plans to purchase Russia’s S-400 air defense system as no final and formal decision has been made yet.

The US has threatened Turkey that it would end the program if it refuses to halt its 2017-finalized deal with Moscow for the advanced missile system.

US will move F-35 work out of Turkey: Pentagon

The Pentagon has threatened to halt manufacturing support for the F-35 planes in Turkey if it buys the Russian S-400 missile system.

Currently, four Turkish pilots are receiving training on F-35 fighter jets at Luke Air Force Base in Arizona and two more are working as instructors at the base. An additional 20 Turkish aircraft maintainers are also participating in the program, according to the US military.

The sources said the decision so far only applied to upcoming rounds of Turkish pilots and maintenance crews who would have normally come to the United States.

On April 1, Washington directly announced that it would be suspending all “deliveries and activities” related to Turkey’s procurement of F-35 stealth fighter jets over Ankara’s plans to purchase the S-400s.

The US claims that the Russian equipment is not “compatible” with the hardware and network lying under the control of the Western military alliance of NATO, which is dominated by Washington and has Turkey as a member.

President Tayyip Erdogan said on Tuesday it was “out of the question” for Turkey to back away from its deal with Moscow.

He added that the US had not “given us an offer as good as the S-400s.”

The Pentagon has declined to comment on the decision, but it has reportedly said talks are underway with Ankara on the potential sale of Patriot missile defenses to Turkey.

Turkish Foreign Minister Mevlut Cavusoglu said on Thursday that the S-400s purchase is a “done deal” and cannot be cancelled.

Cavusoglu highlighted that the United States could not guarantee it would be able to sell the Patriot systems to Ankara due to an impasse in US Congress.

The S-400 is an advanced Russian missile system designed to detect, track, and destroy planes, drones, or missiles as far as over 400 kilometers away. It has previously been sold only to China and India.

Ankara is striving to boost its air defense, particularly after Washington decided in 2015 to withdraw its Patriot surface-to-air missile system from Turkish border with Syria, a move that weakened Turkey’s air defense.

Ankara has been critical of Washington for supporting Kurdish groups in Syria that he says are responsible for terror attacks inside Turkey, among other issues.