Kremlin responds to Biden's warning about using nuclear weapons

This is reported by Russian media. 

In response to the question whether Russia can use nuclear weapons, Peskov offered to read the military doctrine of the Russian Federation.

"Read the doctrine. Everything is written there," Peskov said. 

The military doctrine states that the Russian Federation reserves the right to use nuclear weapons in response to the use of nuclear and other weapons of mass effect against it and its allies, as well as in the event of aggression against the Russian Federation with the use of conventional weapons, when the very existence of the state is threatened.

In June 2020, Vladimir Putin's decree On the Basics of the State Policy of the Russian Federation in the Field of Nuclear Deterrence also lists the conditions under which Russia can use nuclear weapons:

"Received verified information about the launch of ballistic missiles attacking the territory of Russia and/or its allies; the enemy's use of nuclear weapons and other types of weapons of mass destruction in the territories of Russia; the enemy's influence on critically important state or military facilities of Russia, the disabling of which will lead to the disruption of retaliatory actions of nuclear forces; aggression against Russia with the use of conventional weapons, and when the very existence of the state is threatened."