Ukraine's President Volodymyr Zelensky will deliver a virtual address to Congress on Wednesday as the western ally comes under intense shelling from Russia, Democratic leadership in the US Congress said Monday.

"We look forward to the privilege of welcoming President Zelensky's address to the House and Senate and to convey our support to the people of Ukraine as they bravely defend democracy," House leader Nancy Pelosi and her Senate counterpart Chuck Schumer said in a joint letter to lawmakers.

The announcement came with both sides launching a fresh round of talks, more than two weeks after Russia's President Vladimir Putin ordered the invasion, amid deadly air strikes in the capital Kyiv and an eastern pro-Moscow separatist region.

Ukraine said it would demand "peace, an immediate ceasefire and the withdrawal of Russian troops" during the talks.

Pelosi and Schumer said in the letter Zelensky's virtual address would begin at 9:00 am (1300 GMT).

"The Congress remains unwavering in our commitment to supporting Ukraine as they face Putin's cruel and diabolical aggression, and to passing legislation to cripple and isolate the Russian economy as well as deliver humanitarian, security and economic assistance to Ukraine," they added.