Moscow, he said, would view that as the direct participation of Nato countries in the war in Ukraine.
The Kremlin leader continued that it would substantially change the very essence, the nature of the conflict which will mean that Nato countries, the USA and European states, are fighting with Russia.
He claimed that, for missile launches into Russia, Ukraine would require data from Western satellites and that only servicemen from Nato member states would be able to input flight missions into these missile systems.
Russia has drawn red lines before. And seen them crossed before.
On 24 February 2022, when he announced the start of his special military operation – the full-scale invasion of Ukraine – President Putin issued a warning to those who may be tempted to interfere from the outside.
Western leaders ignored what was widely interpreted at the time as nuclear sabre-rattling. The West has since provided Ukraine with tanks, advanced missile systems and, most recently, F-16 American fighter jets.
This year Russia has already accused Ukraine of using American long-range ATACMS missiles to target Crimea, the Ukrainian peninsula annexed by Russia.
Meanwhile, Sir Keir Starmer is in Washington for talks with President Biden. Among the issues the two leaders are expected to discuss is the question of Ukraine and long-range missiles.
Western leaders will need to decide which they consider greater: the risk of escalation of this conflict, or the need to lift restrictions on Ukraine’s use of western missiles.