Russia's President Vladimir Putin issued a stark warning to Western countries, saying that Moscow has the right to hit states that provide weapons to Ukraine to strike Russia.
The Kremlin leader also warned the conflict had taken on "elements of a global character" and stressed Moscow was ready for "any scenario" in its confrontation with Kyiv and the West.
Putin was delivering a surprise address to the nation on state TV after days of escalation in the nearly three-year conflict that has seen Ukraine fire U.S. and U.K.-supplied missiles at Russian territory for the first time.
"We consider ourselves entitled to use our weapons against the military facilities of those countries that allow their weapons to be used against our facilities," Putin said.
"In the event of an escalation of aggressive actions, we will respond just as decisively," he said.
The ex-KGB spy has repeatedly threatened the West throughout the conflict, attacking it for arming Ukraine, trying to thwart his military offensive and hitting Moscow with a barrage of sanctions.
But the warning issued on Thursday evening was one of his starkest and most direct, coming at a pivotal moment in the fighting.
"The conflict in Ukraine has acquired elements of a global character," he said.
"We have always preferred to, and now are ready to resolve all disputed questions with peaceful means but we are also ready for any scenario," Putin said.
"If anyone doubts that, they are wrong. There will always be a response," he added.
Putin also claimed Ukraine's strikes on Russian border territories using ATACMS and Storm Shadow missiles had failed.
Outgoing U.S. President Joe Biden gave Ukraine permission to use the missiles to hit Russian territory earlier this week, after months of demands from Kyiv.
The Biden administration has ratcheted up military aid to Kyiv since Donald Trump won the U.S. presidential election earlier this month.
Trump is a skeptic over American aid to Ukraine and boasted on the campaign trail of being able to secure a peace deal within hours of coming to office.