A close ally of President Vladimir Putin said on Sunday that deliveries of offensive weapons to Kyiv that threaten Russia's territories will lead to a global catastrophe and make arguments against using weapons of mass destruction untenable.
Vyacheslav Volodin, speaker of the Duma - Russia's lower house of parliament - warned that the United States and NATO's support of Ukraine is leading the world to a "terrible war".
"If Washington and NATO countries supply weapons that will be used to strike civilian cities and attempt to seize our territories, as they threaten, this will lead to retaliatory measures using more powerful weapons," Volodin said on the Telegram messaging app.
"Arguments that the nuclear powers have not previously used weapons of mass destruction in local conflicts are untenable. Because these states did not face a situation where there was a threat to the security of their citizens and the territorial integrity of the country."
Western allies pledged billions of dollars in weapons for Ukraine last week, although they failed to persuade Germany to lift a veto on providing German-made Leopard battle tanks, which are held by an array of NATO nations but whose transfer to Ukraine requires Berlin's approval.
Since its invasion on Feb 24, which it has cast as defending itself from an aggressive West, Russia has taken control of parts of Ukraine and has said it will never return them.
Kyiv has said that restoring Ukraine's territorial integrity is not open for negotiation.Volodin's comments followed a similar threat last week by Dmitry Medvedev, Russia's former prime minister and president.
Volodin, 58, has been speaker of the lower house, the State Duma, since 2016, having previously held a senior role in the presidential administration. As a member of Putin's Security Council, he has regular access to the president.
"Deliveries of offensive weapons to the Kyiv regime will lead to a global catastrophe," he said.