Putin has previously said that giving Ukraine the green light on missile use would effectively mean that the U.S. and NATO are "in the war."

"Flight assignments for these missile systems can, in fact, only be entered by military personnel from NATO countries. Ukrainian servicemen cannot do this. And therefore, it is not a question of allowing the Ukrainian regime to strike Russia with these weapons or not. It is a question of making a decision whether NATO countries directly participate in the military conflict or not," Putin said in September

https://www.foxnews.com/world/ukraine-fires-first-barrage-us-made-long-range-missiles-russia-kremlin-says