The two NATO allies have sparred publicly for months over Turkey’s order for Russia’s S-400 air defense system, which Washington says poses a threat to the Lockheed Martin Corp F-35 stealthy fighters, which Turkey also plans to buy.

The United States says Turkey cannot have both, but has avoided taking steps until now to curtail or halt planned training of Turkish pilots in the program, a reprisal that could be seen as an embarrassment in Turkey.

The two US officials, who spoke to Reuters this week on condition of anonymity, left open the possibility the decision could be reversed, perhaps if Turkey altered its plans. They 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.

There has not yet been a formal decision to halt the training of the Turkish pilots and maintenance crews now at Luke Air Force Base in Arizona, the sources said. Still, Reuters reported last week that the step was being seriously considered.

Four Turkish pilots are currently training at Luke. Two additional Turkish pilots are at the US base working as instructors. Beyond those six Turkish officers, there are an additional 20 Turkish aircraft maintainers at the base undergoing training as well, the US military says.

Turkey has expressed an interest in buying 100 of the fighters, which would have a total value of $9 billion at current prices.

If Turkey were removed from the F-35 program, it would be one of the most significant ruptures in recent history in the relationship between the two allies, experts said.

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

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

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

Kathryn Wheelbarger, one of the Pentagon’s most senior policy officials, said last week that Turkey’s completion of the transaction with Russia would be “devastating,” dealing heavy blows to the F-35 program and to Turkish interoperability within the NATO alliance.

“The S-400 is a Russian system designed to shoot down an aircraft like the F-35,” said Wheelbarger, an acting assistant secretary of defense. “And it is inconceivable to imagine Russia not taking advantage of that (intelligence) collection opportunity.”

The Pentagon declined comment on whether it would accept new Turkish pilots. But it has stressed discussions are taking place with Ankara on potentially selling Turkey Patriot missile defenses, which are made by Raytheon Co.

Erdogan said on Tuesday, however, that the United States had not “given us an offer as good as the S-400s.”

Reporter’s code: 50101