Putin updated the "Basic Principles of State Policy of the Russian Federation on Nuclear Deterrence" after Biden's move this week.
The Russian document now says that any aggression from a nonnuclear state with the support of a nuclear state will be treated as a joint attack on Russia.
“The Russian Federation reserves the right to use nuclear weapons in case of aggression with the use of conventional weapons against it,” Kremlin spokesperson Dmitry Peskov said.
Peskov also agreed that a conventional missile attack on Russia could trigger a nuclear response from Russia.
The Biden administration lifted a policy on Sunday that had restricted Ukraine's use of the Army Tactical Missile System or ATACMS (pronounced "attack 'ems") inside Russia.
On Tuesday, Ukraine used the ATACMS for the first time in Russia, striking an ammo depot, according to Russian officials.
Russia's reaction appears to be a major escalation in the nearly three-year war in Ukraine, though it's unclear how much weight is behind the new threat.
Putin has repeatedly threatened to use nuclear weapons throughout the war and has yet to deploy them.
The U.S. has also crossed multiple red lines he has set, including when Ukraine invaded the Russian region of Kursk in early August.
On Capitol Hill, lawmakers expressed concern about escalation but cautioned against Putin's nuclear rhetoric.
"It seems like this can be done safely without broadening the war," said Rep. Brad Sherman (D-Calif.).
Still, there is real concern the war is spiraling out of control, even as President-elect Trump has pledged to end it before he takes office.
— Mike Lillis contributed to this report.
