U.S. Secretary of Defense Mark Esper ruled out any compromise that would let Turkey get American F-35 fighter jets if it keeps the Russian-made S-400 missile defense system.

“I’ve been very clear in both my public comments and privately with my Turkish counterpart,” Esper told reporters in Washington on Wednesday. “It’s either the F-35 or the S-400. It’s not both. It’s not park one in the garage and roll the other one out. It’s one or the other.”

The Trump administration said last month that it wouldn’t let Turkey buy and continue to help build the warplane made by Lockheed Martin Corp. after President Recep Tayyip Erdogan went ahead with his plans to begin receiving the S-400. American officials contend the Russian system could compromise U.S. intelligence and let Moscow gain information about the F-35’s stealth technology.

Esper, who took office in July, pledged to provide regular news briefings as he held the Pentagon’s first such session for reporters in a year.