Russian President Vladimir Putin says Russia would consider an attack from a non-nuclear state that was backed by a nuclear-armed one to be a "joint attack", in what could be construed as a threat to use nuclear weapons in the war in Ukraine.

In a televised meeting with security officials, Putin detailed proposed changes to the country's nuclear doctrine, adding new criteria for launching a nuclear response.  "We see the modern military and political situation is dynamically changing and we must take this into consideration," Putin said, citing "the emergence of new sources of military threats and risks for Russia and our allies."

Putin outlined a potential update, saying that "it is proposed to consider aggression against Russia by any non-nuclear state, but with the participation or support of a nuclear power, as their joint attack on Russia."  

Putin also said that proposed changes "clearly set the conditions for Russia to transition to using nuclear weapons."  "We will consider such a possibility once we receive reliable information about a massive launch of air and space attack weapons and their crossing our state border," the president added.  "I mean airplanes of strategic and tactical aviation, cruise missiles, drones, hypersonic and other aircraft," Putin said.

Russian president’s comments come as Kiev seeks approval to use long-range Western missiles against military sites in Russia.

The BBC says Ukraine's President Volodymyr Zelensky has travelled to the US this week and is due to meet US President Joe Biden in Washington today, where Kiev's request is expected to be top of the agenda.

Ukraine has pushed into Russian territory this year and reportedly wants to target bases inside Russia which it says are sending missiles into Ukraine.

The BBC notes that responding to Putin's remarks, Zelensky's chief of staff Andriy Yermak said Russia "no longer has anything other than nuclear blackmail to intimidate the world".