President Vladimir V. Putin of Russia said Wednesday that he planned to lower the threshold for his country’s use of nuclear weapons, an escalation in the Kremlin’s efforts to deter the United States from expanding its military aid to Ukraine.

Mr. Putin, in a televised meeting with top officials, said Russia would adjust its nuclear doctrine — the document defining when the country could use nuclear weapons — to address “new sources of military threats and risks.”

In particular, Mr. Putin said, Russia would treat “aggression against Russia by any nonnuclear state, but with the participation or support of a nuclear state” as a “joint attack on the Russian Federation.” The language was a clear reference to Ukraine, which does not have nuclear weapons, and the military support it receives from nuclear-armed NATO countries like the United States and Britain.

Mr. Putin also said that Russia would be prepared to use a nuclear weapon even in response to an attack carried out with conventional weapons that creates “a critical threat to our sovereignty.” Ukraine last month pressed a cross-border offensive that captured territory in Russia for the first time since the war began and has been lobbying Washington for permission to use Western missiles for strikes deep into Russian territory.

The debate over the threat of Russian nuclear use has come to a head this week with a visit by President Volodymyr Zelensky of Ukraine to the United States, in which he has kept up that lobbying effort to use Western missiles against targets deep inside Russia. Mr. Putin said this month that such a move would “mean that NATO countries — the United States and European countries — are at war with Russia.”

On Wednesday, the Russian leader said that Moscow would consider nuclear use if it detected the start of a massive launch of missiles, aircraft or drones against it.