MOSCOW -- Sirens wailed across Russia and TV stations interrupted regular programming to broadcast warnings Wednesday as part of sweeping drills intended to test the readiness of the country's emergency responders amid the fighting in Ukraine.

The exercise that started Tuesday follows Ukrainian drone attacks on Moscow and other cities. As the readiness drill went on, the Russian Defence Ministry said air defences shot down 31 Ukrainian drones over border regions early Wednesday.

As part of the drills, TV stations broadcast a notice saying: "Attention everyone! The readiness of the public warning system is being tested! Please remain calm!"

Russian media said the exercise's scenario mentions the increasing danger of a conflict between nuclear powers and simulates a response to a situation in which 70 per cent of housing and all vital infrastructure have been destroyed, wide areas contaminated by radioactive fallout and a general mobilization announced.

The stark storyline echoes Kremlin warnings that Western support for Ukraine has increased the threat of a direct confrontation between Russia and NATO.

Coincidentally, the U.S. federal government on Wednesday is testing its Emergency Alert System, designed to allow the president to speak to the American people within 10 minutes during a national emergency via outlets such as radio and television. It also will send test messages to mobile phone customers in the U.S., according to the Federal Emergency Management Agency.

Dmitry Medvedev, deputy head of Russia's Security Council chaired by President Vladimir Putin, has regularly talked about the growing threat of a nuclear conflict.

Lambasting Western officials who talk about increasing military assistance to Kyiv, Medvedev charged over the weekend that "those imbeciles are actively pushing us to World War III."

Such ominous statements and sweeping emergency drills contrast with the government's efforts to assuage a public increasingly tired of the nearly 20 months of fighting that the Kremlin continues to call its "special military operation."

While regularly criticizing the West over Ukraine, Defence Minister Sergei Shoigu and other members of the military brass have said Russia doesn't need another wave of mobilization because the army has enough volunteer soldiers.

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Associated Press writer Rebecca Santana in Washington contributed.