U.S. officials have said the S-400 cannot co-exist with U.S. equipment, as it can be used to collect intelligence on U.S. systems. They are particularly concerned about Russia using the S-400 to learn about the F-35 fighter jet's advanced capabilities.

"If India chooses to go forward with its purchase of the S-400, that act will clearly constitute a significant, and therefore sanctionable, transaction with the Russian defense sector under Section 231 of CAATSA," [Sen. Bob] Menendez wrote in a recent letter to Austin, referring to the law called Countering America's Adversaries Through Sanctions Act.

During the media briefing, Austin also addressed questions on Afghanistan, noting that President Joe Biden had not yet made a decision about the timing of any potential drawdown even as a May 1 deadline to withdraw all U.S. troops from the country LOOMS.