WASHINGTON — The United States is seriously considering suspending training for Turkish pilots on advanced F-35 fighter jets as Ankara moves ahead with plans to purchase a Russian missile defense system despite objections from Washington, sources told Reuters.

The two NATO allies have argued for months over Turkey's order for the Russian S-400 defenses, which Washington says are incompatible with the Western alliance's defense network and would pose a threat to American F-35 stealth fighters, which Turkey also plans to buy.

The two sources, who are familiar with Turkey's role in the F-35 program and who spoke on condition of anonymity, did not say a final decision had been made.

But the deliberation follows further signs that Turkey is moving ahead with the S-400 purchase. Turkish Defense Minister Hulusi Akar said on May 22 that Turkish military personnel were receiving training in Russia to use the S-400, and said Russian personnel may come to Turkey.

The Pentagon and State Department declined to comment.

The Turkish pilots have been training at Luke Air Force Base in Arizona. It was unclear whether a decision to suspend their training would mean they would also have to leave the country, or if they remain at the base until a final decision is made about Turkey's future in the F-35 program.

The United States has said plainly that Turkey cannot have the S-400 and be part of the F-35 program.

If Turkey was removed from the program, it would be one of the most significant ruptures in recent history in the relationship between the two allies, experts say.

But the strains on ties between Washington and Ankara already extend beyond the F-35 to include strategy in Syria, Iran sanctions and the detention of US consular staff.

On Monday, Turkish broadcaster Haberturk quoted Akar as saying in an interview that the delivery of the S-400 may not happen in June, when Turkey previously said the missiles were due to arrive. However, he added the agreement was a done deal.

"They may not make it by June but they will come in the months ahead. The process has begun," he was quoted as saying. The comments lifted Turkey's lira currency, battered this year by the disagreement between Ankara and Washington.

Objecting to Ankara's planned Russian defense purchase, the United States in late March halted delivery of equipment related to the F-35 to Turkey, which is both a buyer and a production partner in the program.

The United States has warned that if Turkey takes delivery of the Russian system, it will also trigger US sanctions under CATSAA, a law calling for sanctions against countries procuring military equipment from Russia.

Turkey has said that as a NATO member it poses no threat to the United States and the sanctions should not apply. Ankara has also increasingly pinned its hopes on President Donald Trump to protect it from such penalties.

US officials have called Turkey's planned purchase of the S-400 system "deeply problematic." Washington and other NATO allies that own F-35s fear the system's radar will learn how to spot and track the jet, made by Lockheed Martin Corp, making it less able to evade Russian weapons. — Reuters