UNITED NATIONS, UNITED STATES (AFP) - The United States ambassador to the United Nations accused Russia on Wednesday (March 2) of moving cluster munitions and other arms banned under the Geneva Convention into Ukraine as Moscow seeks to escalate the attack on its eastern European neighbour.

Linda Thomas-Greenfield spoke as Russian forces shelled several Ukrainian cities on Wednesday and massed outside the capital Kyiv, raising fears of an imminent assault.

"It appears Russia is preparing to increase the brutality of its campaign against Ukraine," Thomas-Greenfield told the United Nations General Assembly.

"We have seen videos of Russian forces moving exceptionally lethal weaponry into Ukraine, which has no place on the battlefield. That includes cluster munitions and vacuum bombs - which are banned under the Geneva Convention."

She also cited published images of a 65km-long convoy of Russian troops which she said were "charging towards Kyiv."

Thomas-Greenfield spoke ahead of an extraordinary vote which saw the General Assembly demand Russia withdraw its forces from Ukraine "immediately."

An overwhelmingly majority of nations at the global body voted in favour of the non-binding resolution deploring Moscow's attack, in a powerful indicator of Russia's isolation on the world stage.

Russia has defied massive economic and diplomatic sanctions to push on into pro-Western Ukraine, where its forces have encountered stiff resistance.

Follow The Straits Times' live coverage on the Ukraine crisis here.