The United States was warned Friday it cannot unilaterally drop NATO ally Turkey from the F-35 fighter jet program as the partnership agreement specifically forbids it, Turkey’s head of Defense Industries Directorate said.

“No single country can say they don’t want you and then remove you from the program,” Ismail Demir told the Reuters newsagency.

“This isn’t part of the agreement, this isn’t something you can just say ‘I exclude you’ about. The F-35 project is a partnership and nowhere in the agreement does it allow a unilateral removal of one country,” he added.

The U.S. has previously warned Turkey it would refuse to sell its high-tech F-35 warplane and other weapons systems if Ankara moves forward with the purchase of the S-400 missile system from Russia.

In April it delivered on that, suspending all deliveries until further notice.

Washington has since stopped accepting more Turkish pilots for training in the U.S. and halted delivery of equipment related to the program.

U.S. Vice President Mike Pence also declared on February 16 the Trump administration has “made it clear that we will not stand idly by while NATO allies purchase weapons from our adversaries.”

Reuters reports the U.S. has looked to impose sanctions on Ankara under its Countering America’s Adversaries Through Sanctions Act (CAATSA), the possibility of which has spooked investors and helped cause a selloff in the lira this year.

Demir said such sanctions could have a brief impact on Turkey’s defense industry. “Our defense industry produces parts for the F-35, so in the event of sanctions being imposed, our industry would experience a rough patch, but we’ll then get past this,” he said.