Ukrainian officials said some 140,000 people had been evacuated from cities under threat from Russian attack, but no such operation has yet been launched in the besieged southern port of Mariupol owing to constant shelling. The death toll in the city was put at 2,100.

More than 12,000 residents were evacuated from several towns, but not from Mariupol, surrounded for more than a week, its residents deprived of heating, water and running dangerously low on food.

Biden pointedly drove home the fundamental U.S. retaliatory plan to Russia's invasion - Washington was prepared to do everything to defend NATO from attack, but would not risk triggering “World War Three” by engaging directly with Russia.