LONDON -- Kremlin spokesperson Dmitry Peskov told journalists that new changes to Russia's nuclear weapons doctrine signed by President Vladimir Putin on Tuesday mean "the use of Western non-nuclear rockets by the Armed Forces of Ukraine against Russia can prompt a nuclear response."

Peskov's remarks came shortly after three U.S. officials confirmed to ABC News that President Joe Biden had approved Ukraine's use of the long-range American-made MGM-140 Army Tactical Missile System -- colloquially known as the ATACMS -- to hit targets in Russia's western Kursk region.

The administration hasn't publicly confirmed the policy change. State Department spokesperson Matthew Miller told journalists at a Monday briefing he would not confirm or deny approval for ATACMS use inside Russia, but said the U.S. response to Russian and North Korean military cooperation in the war "would be firm."

There are now some 10,000 North Korean troops in Russia's Kursk region intended for deployment to the battlefield, U.S. officials have said.

The changes to Russia's nuclear doctrine were unveiled several weeks ago but only signed by Putin on Tuesday, as officials in Moscow expressed anger at the U.S. decision to allow ATACMS use on Russian territory.

The doctrine now says Russia can launch a nuclear attack against a country assisting a non-nuclear country in aggression against Russia that critically threatens the country's state integrity.

Moscow has repeatedly threatened nuclear weapon use against Ukraine and its Western partners throughout its full-scale invasion of the country.

Western leaders including President Joe Biden have said that avoiding a direct clash between Russia and NATO is a top priority given the danger of nuclear war.

This is a developing story. Please check back for updates.