WASHINGTON (AP) — The top U.S. commander for Europe says he would recommend the United States not sell its high-tech F-35 fighter jet to Turkey, if Ankara moves ahead with plans to buy a Russian missile defense system.

Gen. Curtis Scaparrotti, head of U.S. European Command, says Turkey should reconsider its plan to buy the S-400 from Russia this year or forfeit other future American military aircraft and systems. He says the S-400 surface-to-air missile defense system would be a threat to the F-35.

Scaparrotti says U.S. officials are currently in Turkey explaining the consequences of the S-400 purchase.

The State Department in December approved the sale of a $3.5 billion Patriot missile defense system to Turkey. The F-35 delivery to Turkey has been delayed as negotiations about the S-400 continue.