Emergency workers remove debris of a building destroyed in Mariupol, Ukraine, on Sunday. Reuters

Ukraine's armed forces braced on Monday for a new Russian offensive as powerful explosions rocked cities in the south and east, while Austria's leader planned to meet Russian President Vladimir Putin and call for an end to the conflict.

A showdown looms in Ukraine as Russia appointed a new military commander and looked to concentrate its attacks on the eastern part of the country, while Ukraine’s president said his troops will hold their ground, and urged Western leaders, in particular President Joe Biden, to do more.

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Air raid sirens were heard across Ukraine early on Monday.

"It is likely that the enemy, in order to disrupt the supply of goods to the places of hostilities, will continue to strike at transport infrastructure facilities in Ukraine in order to destroy or disable them,” the general staff of Ukraine's armed forces said.

Russian forces were continuing their offensive to establish full control over the southern city of Mariupol, seeking to storm an iron and steel plant and the seaport, it added.

Residents walk near a building destroyed in the course of the Ukraine-Russia conflict in Mariupol. Reuters

Austrian Chancellor Karl Nehammer said he would meet Putin on Monday in Moscow for the Russian leader's first face-to-face meeting with a European Union counterpart since Russia's invasion began on Feb. 24.

"We are militarily neutral, but (have) a clear position on the Russian war of aggression against #Ukraine," Nehammer wrote of Austria on Twitter. "It must stop! It needs humanitarian corridors, ceasefire & full investigation of war crimes."

Ukrainian President Volodymyr Zelensky warned on Sunday in his nightly address to the nation that the coming week would be as crucial as any during the war, saying "Russian troops will move to even larger operations in the east of our state.”

Service members of pro-Russian troops drive armoured vehicles on a road outside Mariupol, Ukraine. Reuters

Zelensky kept up his tireless campaign to generate international support and rally his countrymen, warning the coming week would be important and tense.

Zelensky also said that Ukraine’s fate as the war shifts south and east depends on whether the United States will help match a surge in Russian weaponry in the regions.

"To be honest, whether we will be able to (survive) depends on this,” said Zelensky said in a "60 Minutes” interview that aired Sunday night, speaking through a translator. "Unfortunately, I don’t have the confidence that we will be receiving everything we need.

Zelensky said he was grateful to Biden for US military aid to date but added that he "long ago” forwarded a list of specific items Ukraine desperately needed and that history would judge Biden’s response.

Cemetery workers remove a body from a mass grave, for identification in a morgue, in Bucha on Sunday. AP

"He has the list,” Zelensky said. "President Biden can enter history as the person who stood shoulder to shoulder with the Ukrainian people who won and chose the right to have their own country. (This) also depends on him.”

"Russia will be even more afraid. It will be afraid to lose. It will fear that the truth will have to be acknowledged," Zelensky said in a late night video address.

"Russian troops will move to even larger operations in the east of our state. They may use even more missiles against us, even more air bombs. But we are preparing for their actions. We will answer."

Experts have said that the next phase of the battle may begin with a full-scale offensive. The outcome could determine the course of the conflict, which has flattened cities, killed untold thousands and isolated Moscow economically and politically.

A resident looks on near a building destroyed in the course of the Ukraine-Russia conflict, in Mariupol. Reuters

Questions remain about the ability of Russia’s depleted and demoralized forces to conquer much ground after their advance on the capital, Kyiv, was repelled by determined Ukrainian defenders. Britain’s Defense Ministry reported Sunday that the Russian forces were trying to compensate for mounting casualties by recalling veterans discharged in the past decade.

In his Sunday night address, Zelensky also accused Russia of trying to evade responsibility for war crimes in Ukraine.

"When people lack the courage to admit their mistakes, apologize, adapt to reality and learn, they turn into monsters. And when the world ignores it, the monsters decide that it is the world that has to adapt to them,” Zelensky said.

"The day will come when they will have to admit everything. Accept the truth,” he added.