Russian President Vladimir Putin warned Thursday that a decision by the U.S. or its NATO allies to allow Ukraine to use Western-supplied long-range missiles to hit targets deep inside Russia would be viewed as NATO’s direct participation in the war, which he said would significantly change “the very nature of the conflict.”

“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 response to a question on Thursday.

“If this decision is made, it will mean nothing less than the direct participation of NATO countries, the United States, and European countries, in the war in Ukraine,” the Russian leader added.

Ukrainian President Volodymyr Zelenskyy has been asking his Western partners to allow his country to fire long-range missiles supplied by the U.S. or Europe deep into Russian territory, including U.S. ATACMS and British Storm Shadows.

President Biden is expected to discuss the subject in a meeting with U.K. Prime Minister Keir Starmer in Washington, D.C., later on Friday.

If the U.S. and Britain approve the move, it “will mean that NATO countries, the United States, and European countries are fighting Russia,” Putin said. “And if this is so, then, bearing in mind the change in the very essence of this conflict, we will make appropriate decisions based on the threats that will be created for us.”

When asked Friday whether the Kremlin had seen a reaction among Western powers to Putin’s statement, spokesperson Dmitry Peskov said “the statement that Putin made yesterday is very important. It is extremely clear, unambiguous and does not allow for any double readings. And we have no doubt that this statement has reached its addressees.”