Bloomberg – President Recep Tayyip Erdogan broke his silence over a U.S. threat to punish Turkey with sanctions should it buy an advanced Russian S-400 missile-defense system, saying the purchase is a “done deal.”

“It is out of the question for us to revoke the S-400 deal,” Erdogan said during an interview with TV24 television broadcast late Wednesday. “Such an immoral act would not suit us.”

The U.S. warned this week that it could retaliate by blocking Turkey’s planned purchase of the next-generation F-35 fighters it’s helping to build. The U.S. fears the Russian system could be used to collect intelligence on the jet’s stealth capabilities, but Erdogan said “we’ve researched it, there is no such thing.”