US Treasury Secretary Steve Mnuchin has warned Turkey that it may be sanctioned over its purchase of Russian-made S-400 missile defense systems. While no decision has yet been made, Mnuchin stressed that the White House was “looking at” penalizing its NATO ally. The US believes that the S-400s, which are not interoperable with NATO systems, could compromise its F-35 stealth fighter jets while allowing Moscow to access sensitive American technologies. In July, US President Donald Trump announced that Ankara would be banned from the F-35 program. Turkey had ordered more than 100 F-35s at an estimated cost of $1.4 billion.