Putin spoke as US and UK top diplomats discussed easing rules on firing Western weapons into Russia, which Kyiv has been pressing for, more than two and a half years into Moscow's offensive.

"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."

Clearing Kyiv 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 Kyiv pressed the West to provide more powerful weapons with fewer restrictions, as Russia continues its advance into eastern Ukraine.