UKRAINE'S President Volodymyr Zelenskiy said on Monday that Ukrainian intelligence believed Russian forces were preparing a new, imminent mass attack on the country.

Zelenskiy, speaking in his nightly video address, suggested the new attack could occur on Monday, but gave no further details of a time frame or possible targets.

"Please pay attention to air raid alerts today. Intelligence indicates that Russia may be preparing a massive strike," he said. "Relevant orders for air defence have already been issued."

Russian forces have regularly staged air strikes on Ukrainian towns and cities, mainly at night and including the capital Kyiv. Energy infrastructure has been a repeated target.