The top US diplomat told his Turkish counterpart that Turkey’s purchase of a Russian air defense system “endangers” US security forces on a recent call.

US Secretary of State Mike Pompeo spoke to Turkish Foreign Minister Mevlut Cavusoglu, a State Department spokesperson said on Thursday. Pompeo told Cavusoglu that Turkey’s purchase of the Russian S-400 missile defense system is a threat to the United States.

“Secretary Pompeo made clear to Foreign Minister Cavusoglu that Turkey’s purchase of the S-400 system endangers the security of U.S. personnel and military technology and allows Russian access to the Turkish armed forces and defense industry,” said the State Department spokesperson in a statement.

Turkey purchased the S-400 missile defense system from Russia in 2019 over objections from the United States. Both Turkey and the United States are part of the trans-Atlantic NATO military alliance. Russia is not, and opposes NATO on many issues in eastern Europe in particular. NATO members have signed a collective defense pledge as well as agreements on “harmonizing defense and arms control policies.”