Republican Speaker Mike Johnson has held up a bill for months that would supply $US60 billion ($A92 billion) in military and financial aid for Ukraine.

"Quick passage of US aid to Ukraine by Congress is vital. We recognise that there are differing views in the House of Representatives on how to proceed, but the key is to keep the issue of aid to Ukraine as a unifying factor," Zelenskiy said on X.

In his nightly video address, Zelenskiy said he told Johnson that Russian strikes on Ukrainian cities were "escalating and can only be stopped by the physical force of our defence".

"And it is very important for Congress to maintain its leadership so that the defence of freedom remains an idea that unites - within our countries and in a majority in the world."

Ukrainian troops are on the back foot on the battlefield, facing shortages of artillery supplies with the US assistance held up in Congress and the European Union failing to deliver on time munitions that it had promised earlier.

The Ukrainian military said that its top commander, Oleksander Syrskyi, had spoken to the US Chairman of Joint Chiefs of Staff, Charles Brown, about battlefield issues.

An account of the conversation, on Telegram, said Syrskyi discussed "the question of vital US help for Ukraine", including strengthening defences against Russian air attacks and building fortifications.

Last Friday, Russia conducted its largest air strike on Ukraine's energy system since invading in February 2022, damaging power units at a major dam and causing blackouts for more than a million people.

Moscow has described its recent attacks as part of a series of "revenge" strikes in response to Ukrainian attacks on Russian regions.

Russia has increased its use of harder-to-stop ballistic missiles. It denies targeting civilians, though many have been killed in its strikes.

Australian Associated Press