Ukraine on Tuesday fired U.S.-supplied long-range missiles into a warehouse used to store weapons in the Bryansk region of Russia, aggravating a war that could make President Vladimir Putin respond aggressively with nuclear bombs.

Russia’s Ministry of Defence confirmed that Ukrainian forces fired six Army Tactical Missile Systems, or ATACMS on Tuesday but said five of the warheads were shut down and that the sixth was damaged enough to not cause significant harm.

Moscow, however, said the fragmented pieces of the ATACMS burst into flames at a military facility, adding that no casualty was recorded. 

The U.S. supplied Ukraine with ATACMS in 2023 but withheld permission to use it against Russia over fears that it may escalate the war into a full-blown World War III.

But Russia teaming up with North Korea against Ukraine has shattered President Joe Biden’s last shred of restraint, authorising that the warheads —which can harm targets as far as 190 miles— be fired into Russian territory on Sunday.

On his part, Mr Putin lowered the threshold for firing nuclear weapons at a time that coincided with U.S.’s authorisation for ATACMS, as if to send a clear message that he was prepared to aggressively go all-out to war.

Mr Putin’s spokesperson, Dmitry Peskov, said that Russia “reserves the right” to use its nuclear ammunition to shield its people from weapons that constitute “critical threat.” 

Tuesday’s revised wording of the arms’ doctrine  stated that the Russian forces were free to use nuclear warheads against attacks from any country backed by a nuclear state. 

“Aggression against the Russian Federation and (or) its allies by any nonnuclear state with the participation or support of a nuclear state is considered as their joint attack,” stated the document.

“Nuclear deterrence is aimed at ensuring that a potential adversary understands the inevitability of retaliation in the event of aggression against the Russian Federation and/or its allies,” said Mr Peskov.