The US official made the comments outside the White House when asked by reporters if the Treasury was considering such sanctions, where he also pointed out that Washington, however, has not made any decisions yet.
"We're looking at that, I'm not going to make any comments on any specific decisions, but we are looking at it," Mnuchin said, as quoted by Reuters.
The US has been concerned over Turkey's purchase of the S-400 system, saying that it is not compatible with the NATO defense.
Ankara received the Russian air defense system in July as part of a previous agreement with Moscow. The US has repeatedly threatened to sanction Turkey if it bought the system, but it has not made such moves so far.
Reporter's code: 50101