Turkish President Recep Tayyip Erdogan said Wednesday it was “out of the question” to reconsider a controversial deal to buy a top-line Russian anti-missile system and he might even look at other purchases.

The United States, a long-standing NATO ally and supplier of much of Turkey’s military equipment, has pressed Ankara to drop the S-400 deal, saying it is incompatible with its established weapon systems and is part of Russian efforts to muddy the waters in a key, volatile region.

“As for the S-400, the deal is done and it is out of the question to go backwards… and perhaps after the S-400, we will move onto the S-500,” Erdogan told Kanal 24 television.

The S-400 is a sophisticated anti-missile and anti-aircraft weapon system, similar to the US Patriot.

Erdogan also noted Washington’s decision to withdraw trade privileges from Turkey, warning that “no one should try to make us toe the line with such measures.”