"If such decision has indeed been formulated and communicated to the regime in Kiev, this is a qualitatively new round of tension and a qualitatively new situation regarding the involvement of the United States in this conflict," Kremlin spokesman Dmitry Peskov told reporters, quoted by the BBC.
On Sunday, the New York Times and other media reported that US President Joe Biden lifted the ban on using US long-range ATACMS missiles to strike targets in Russia. According to journalists, the first stage involves targets in the Kursk border region, which is partially occupied by Ukrainian armed forces.
Responding to a question about Moscow's reaction to Biden's decision, Peskov recalled that Russian President Vladimir Putin had already made a statement on this topic in September and warned of countermeasures.
"Flight missions for these missile systems can essentially only be carried out by military personnel of NATO countries," Putin said on September 12. "Ukrainian military personnel cannot do this.
"If this decision is taken, it will mean nothing less than the direct participation of NATO countries - the USA, European countries - in the war in Ukraine,", the Russian president said in September. This will mean that NATO countries, the USA, European countries are at war with Russia.
Putin did not say exactly what the reaction might be, framing his warning as follows: "We will take appropriate decisions based on the threats that will be created for us.