Civilians were fleeing from areas of eastern Ukraine on Tuesday ahead of an anticipated Russian offensive, while Kyiv said it was checking reports that Russian forces had used chemical weapons in the besieged port city of Mariupol.

The battle for Mariupol was reaching a decisive phase, with Ukrainian marines holed up in the Azovstal industrial district.

Should the Russians seize Azovstal, they would be in full control of Mariupol, the lynchpin between Russian-held areas to the west and east. The city has already been laid waste by weeks of Russian bombardments that have killed possibly thousands of civilians.

Deputy Defence Minister Hanna Malyar said the government was checking unverified information that Russia may have used chemical weapons while besieging Mariupol.

"There is a theory that these could be phosphorous munitions," Malyar said in televised comments.

President Volodymyr Zelenskiy had said on Monday night that Russia could resort to chemical weapons as it amassed troops in the eastern Donbas region for a new assault on Mariupol. He did not say if they actually had been used.

The United States and Britain said they were trying to verify the reports. If Russia had used chemical weapons, "all options were on the table" in response, British Junior Defence Minister James Heappey said in London.

Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention (CWC). Although condemned by human rights groups, white phosphorous is not banned under the CWC.

The Russian defence ministry has not yet responded to a Reuters request for comment. Russian-backed separatist forces in the east denied using chemical weapons in Mariupol, the Interfax news agency reported.

But should it prove to be the case, it would mark a dangerous new development in a war that has already left a trail of death of destruction since Russian President Vladimir Putin sent his troops over the border on Feb. 24.

About a quarter of Ukraine's 44 million population have been forced from their homes, cities turned into rubble, and thousands of people have been killed or injured - many of them civilians.

REDOUBLING EFFORTS

Putin calls the action a "special military operation" to demilitarise and "denazify" Ukraine but it has drawn condemnation and alarm in the West, which has imposed a wide range of sanctions to squeeze the Russian economy.

After their troops got bogged down in the face of Ukrainian resistance, the Russians abandoned their bid to capture the capital Kyiv for now. But they are redoubling their efforts in the east and Ukrainian forces are digging in to face a new offensive.

The governor of Luhansk region, Serhiy Gaidai, urged residents to evacuate using five humanitarian corridors agreed for the east.

"It's far more scary to remain and burn in your sleep from a Russian shell," he wrote on social media. "Evacuate, with every day the situation is getting worse. Take your essential items and head to the pickup point."

In all, nine humanitarian corridors had been agreed for Tuesday, including one for private cars from Mariupol, Deputy Prime Minister Iryna Vereshchuk said.

In its morning briefing on the conflict, the General Staff of Ukraine’s Armed Forces said that aside from trying to take control of Mariupol, Russian forces were also intent on capturing Popasna, a town about two hours drive west of Luhansk, and were set to launch an offensive in the direction of Kurakhove, in the Donetsk region.

The Ukrainian military said its troops had repulsed attacks in both Luhansk and Donetsk.

President Zelenskiy pleaded overnight for more weapons from the West to help it end the siege of Mariupol and fend off the expected Russian offensive in the east.

"Unfortunately we are not getting as much as we need to end this war faster... in particular, to lift the blockade of Mariupol," he said.

WAR CRIMES ALLEGATIONS

The departure of Russian forces from the outskirts of Kyiv brought to light allegations of war crimes including executions and rape of civilians. Moscow dismisses the allegations as Ukrainian and Western provocations and has also accused Ukrainian forces of sexual violence.

Senior U.N. official Sima Bahous told the U.N. Security Council on Monday that while all allegations must by independently investigated, "the brutality displayed against Ukrainian civilians has raised all red flags".

"We are increasingly hearing of rape and sexual violence," she said.

Kateryna Cherepakha, president of rights group La Strada-Ukraine, told the council via video: "Violence and rape is used now as a weapon of war by Russian invaders in Ukraine."

Russia's deputy U.N. ambassador denied the allegations and accused Ukraine and allies of "a clear intention to present Russian soldiers as sadists and rapists".

Russia's defence ministry said Ukraine's government was being directed by the United States to sow false evidence of Russian violence against civilians despite what it cast as Moscow's "unprecedented measures to save civilians".

Putin is scheduled to meet Belarusian President Alexander Lukashenko on Tuesday to discuss Ukraine and Western sanctions, news agencies in Russia and Belarus reported. Belarus is a key staging area for Russian forces.

The Russian leader said on Tuesday that Moscow's military operation would undoubtedly achieve what he said were its "noble" objectives.

Speaking at an awards ceremony at the Vostochny Cosmodrome in the Russian Far East, Putin was quoted as saying by Russian news agencies that Moscow had no other choice but to act to protect Russia and that a clash with Ukraine's anti-Russian forces had been inevitable.