President Vladimir Putin put Russia's nuclear deterrent on high alert on Sunday in the face of a barrage of Western reprisals for his war on Ukraine, which said it had repelled Russian ground forces attacking its biggest cities.

The United States said Putin was escalating the war with "dangerous rhetoric", amid signs that the biggest assault on a European state since World War Two was not producing rapid victories, but instead generating a far-reaching and concerted Western response.

Less than four days after it started, the invasion has triggered a Western political, strategic, economic and corporate response unprecedented in its extent and coordination.

"With this war on Ukraine...