-Russia's foreign ministry said on Monday it had summoned U.S. Ambassador John Sullivan to tell him that remarks by President Joe Biden about Russian President Vladimir Putin had pushed bilateral ties to the brink of collapse.
President Biden said last week that Putin was a "war criminal" for sending tens of thousands of troops into Ukraine.
"Such statements from the American president, unworthy of a statesman of such high rank, put Russian-American relations on the verge of rupture," the ministry said in a statement.
The Kremlin earlier described the comments as "personal insults" against Putin.
The ministry also told Sullivan that hostile actions against Russia would receive a "decisive and firm response".
Russia sent tens of thousands of troops into Ukraine on Feb. 24 in what it called a special operation to degrade its southern neighbour’s military capabilities and root out people it called dangerous nationalists.
Ukrainian forces have mounted stiff resistance against Russian forces and the West has imposed sweeping sanctions on Moscow in an effort to force it to withdraw.