President Volodymyr Zelensky said protecting Ukraine's borders was a "constant priority" and his country was ready for all possible scenarios with Russia and its ally Belarus, which Kyiv has warned could be drawn into the 10-month conflict.

Zelensky, in his nightly video address to Ukrainians, also issued a fresh appeal to Western nations to provide Kyiv with better air defences as "one of the most powerful" steps to halt the Russian invasion.

Early on Monday, air raid warnings again rung out over Kyiv and eastern Ukraine, with videos of blasts and air defence systems shared on social media.

"Protecting our border, both with Russia and Belarus - is our constant priority," Zelensky said after a meeting on Sunday of Ukraine's top military command. "We are preparing for all possible defence scenarios."

Zelensky was speaking on the eve of a visit to Belarus by Russian President Vladimir Putin amid discussion of a possible new Russian offensive and suggestions it could originate in Belarus.

Officials in Kyiv have warned for months that Belarus could join Russian forces and serve as a launching pad for a new attack to form a second front in the war.

Whatever Belarusian President Alexander Lukashenko might be persuaded to do for Russia "this will not help them, just like all the other sick ideas in this war against Ukraine and Ukrainians," Zelensky said.

Belarus is one of Russia's closest allies and Lukashenko allowed his country's territory to be used to embark on the Feb 24 invasion of Ukraine. But he has said repeatedly that he has no intention of sending his country's troops into Ukraine.

Kissinger calls for negotiations

Putin casts what he calls Russia's "special military operation" as a watershed moment when Moscow finally stood up to a Western bloc, led by the US, seeking to capitalise on the 1991 fall of the Soviet Union by destroying Russia.

Kyiv and the West say Putin has no justification for what they have decried as an imperial-style war of occupation that has resulted in Russia now controlling around a fifth of Ukraine.

Henry Kissinger, an architect of the Cold War policy of detente towards the Soviet Union as secretary of state in the 1970s, said the time was approaching for a negotiated peace.

"The time is approaching to build on the strategic changes which have already been accomplished and to integrate them into a new structure towards achieving peace through negotiation," Kissinger wrote in The Spectator magazine.

Ukraine rejected the proposal, saying it amounted to appeasing the aggressor by sacrificing parts of Ukraine.

"All supporters of simple solutions should remember the obvious: any agreement with the devil - a bad peace at the expense of Ukrainian territories - will be a victory for Putin and a recipe for success for autocrats around the world," Ukrainian presidential aide Mykhailo Podolyak said on Telegram.

Kremlin officials were not available for comment late on Sunday.

New appeal for air defence system

Zelensky has called for a global peace summit to help resolve the conflict, and had hoped to broadcast a message ahead of the soccer World Cup final. That request had been declined by soccer officials, he said, but the world still had heard his call for peace.

In his remarks, Zelensky also issued the latest of many calls for Western nations to beef up Ukraine's air defences after weeks of Russian air strikes on the country's energy network.

Zelensky said power had been restored to three million more Ukrainians in the past 24 hours following last Friday's mass missile attack on electricity infrastructure that killed three people and damaged nine power facilities.

"Electricity supplies have been restored to a further three million Ukrainians," he said. "Plus six million yesterday. That means after the terrorist strikes on Friday, we have results already for nine million of our people."

The conflict has killed tens of thousands of people and driven millions from their homes.

Zelensky told Ukrainians the armed forces were holding firm in the town of Bakhmut - scene of the fiercest fighting in the country for many weeks as Russia attempts to advance in eastern Ukraine's Donetsk region.

"The battlefield in Bakhmut is critical," he said. "We control the town even though the occupiers are doing everything so that no undamaged wall will remain standing."

Denis Pushilin, Russian-installed administrator of the portion of the Donetsk region controlled by Moscow, said that Ukrainian forces shelled a hospital in the Donetsk city, killing one person and injuring several others.