"The S-400, once activated, will increase the threat to our forces and our coalition partners flying over Syria," he said. "We are still working out on how it is going to be executed."
Russia has deployed S-400 mobile missile batteries at the Khmeimim Airbase in Syria but has not yet activated the defence systems.
The S-400 is the next-generation mobile defence system which can carry three different types of missiles capable of destroying a variety of aerial targets at short and extremely long ranges, from reconnaissance aircraft to ballistic missiles.
Balance in Syria
McKenzie also stressed that Washington would not seek to affect the Russian forces in the region while conducting operations against terrorists.
"It is clearly not an objective of our presence in Syria," McKenzie said during Senate testimony when asked whether the goal of the US military presence in Syria was to serve as a check against Russia's influence there.
*Daesh, also known as IS/ISIL/ISIS/Islamic State, is a terrorist organisation banned in Russia