President Vladimir Putin say the West letting Kiev use longer-range weapons to strike Russian targets would mean NATO would be "at war" with Russia.
Putin spoke as US and UK top diplomats discussed easing rules on firing Western weapons into Russia, which Kiev has been pressing for, more than two and a half years into the war.
"This would in a significant way change the very nature of the conflict," Putin told a state television reporter.
"It would mean that NATO countries, the US, European countries, are at war with Russia," he added.
"If that's the case, then taking into account the change of nature of the conflict, we will take the appropriate decisions based on the threats that we will face."
Putin said clearing Kiev to strike deep into Russia "is a decision on whether NATO countries are directly involved in the military conflict or not".
Putin's comments came as Kiev pressed the West to provide more powerful weapons with fewer restrictions, as Russia continues its advance into eastern Ukraine.
'Russian forces push back Ukraine troops in Kursk'
A senior commander said Thursday Russian forces had regained some areas in Kursk from Ukrainian troops, more than a month after the Ukrainian army launched an incursion into Russia’s eastern region.
Russian troops had gone on the offensive and liberated some 10 settlements in Kursk, said Major General Apti Alaudinov, who commands Chechnya’s Akhmat Special Forces fighting in Kursk.
“The situation is good for us. A total of 10 settlements in the Kursk region have been liberated,” said Alaudinov, who is also deputy head of the Russian defense ministry's military-political department.
He did not give further details but TASS news agency, in a report, identified these areas as Apanasovka, Byakhovo, Vishnevka, Viktorovka, Vnezapnoye, Gordeyevka, Krasnooktyabrskoye, Obukhovka, Snagost and Tenth October.
On August 6, Ukraine executed the largest foreign assault on Russia since World War II, breaching the border into Kursk with thousands of troops backed by numerous drones and heavy weaponry, including Western-made arms.
Ukrainian President Volodymyr Zelensky said last week that his forces held 100 settlements in the Kursk region, covering over 1,300 square kilometers. However, Russian sources have disputed his claim.
Yuri Podolyaka, a pro-Russian military blogger of Ukrainian origin, along with two other prominent bloggers, Rybar and the Two Majors, reported that Russian forces had initiated a major counteroffensive in Kursk.
“In the Kursk region, the Russian Army launched counter-offensive actions on the western flank of the enemy’s wedge, reducing the Ukrainian zone of control near the state border,” the Two Majors blog said.
According to Podolyaka, Russian forces have liberated several villages west of the area Ukraine seized, pushing Ukrainian troops east of the Malaya Loknya River, south of Snagost.
Russia launched its “special military operation” in Ukraine’s Donbas on February 24, 2022. Since then, Western countries have been supplying the former Soviet republic with military equipment, turning the conflict into a full-fledged war, despite repeated warnings by the Kremlin that such a policy will only prolong the war.