The speaker of Russia's Duma, the lower house of parliament, Vyacheslav Volodin, has accused NATO of being a party to the military conflict in Ukraine, suggesting the pact is already heavily involved in military decision-making.

Volodin, a close aide to President Vladimir Putin, made the comments just a day after Putin warned that the West would fight Russia directly if it allowed Ukraine to attack Russian territory with Western long-range missiles, a move he said would will change the nature of the conflict.

Without offering any evidence to back up his claims, Volodin accused the US-led alliance of helping Ukraine choose which Russian cities to target by orchestrating specific military actions and issuing orders to Kiev.

"The USA, Germany, Great Britain and France are discussing the possibility of an attack (on Ukraine) using long-range weapons on the territory of our country. It is nothing but an attempt to camouflage and cover up their direct participation in the military action," Volodin wrote on his Telegram channel.

"With this, the US and its allies are actually trying to give themselves permission to carry out acts of aggression with missiles against Russia," he added.

He said that the activities of NATO advisers and instructors in Ukraine are now supplemented by "mercenaries", as he called them, and spoke of entire units armed with NATO weapons.

The British media could not independently confirm the claims of the Russian politician and close associate of Putin.