Turkish pilots and maintainers have arrived at Luke Air Force Base in Arizona to begin training on their first two F-35 fighter aircraft ― but the jets themselves will remain in U.S. custody for at least the next year, the Pentagon said Monday.

The jets were flown by Lockheed Martin last week to the Arizona base, which is home to the F-35 international partner training program. Turkey’s pilots will likely take their first flight in them in about a month, after they complete some initial classroom and simulator training, said Air Force Lt. Col. Mike Andrews, a Pentagon spokesman. Completion of full pilot training, which is a condition of Turkey assuming custody of the jets takes between one to two years, Andrews said.

But Turkey may not get the jets even after training is complete. The Senate passed language in its version of the 2019 defense authorization bill that would block the jets’ sale to Turkey based on U.S. concerns over Turkey’s purchase of a Russian S-400 long-range air and missile defense system. The House would still need to agree to the language in the final version of the bill.

Both options, to transfer the jet or block it, have potential negative consequences. If Turkey obtains custody, it could lead to Russia gaining sensitive technical information about the aircraft through the S-400 system. If the F-35 transfer is blocked, it could agitate the NATO ally to the point that use of Turkey’s Incirlik Air Base is restricted. The U.S. has depended on Incirlik to conduct its air campaign against the Islamic State in Syria.

“Following established agreements, the U.S. government maintains custody of the aircraft until custody is transferred to the partner,” said Army Col. Rob Manning, a Pentagon spokesman. “The U.S. government has not made a determination on Turkey’s future participation in the F-35 Joint Strike Fighter program.”

Turkey has been a partner in the program since 2002 and has plans to acquire 100 of the fighters.

Until clear direction comes from the White House or Congress, the program will continue, Andrews said.

“The department is not going to comment on proposed legislation,” he said. “Until something changes, they are going to continue their flight academics at Luke.”