President Vladimir Putin on Tuesday made a defiant case for the war on Russia's neighbour, as Russian troops intensified a campaign to take the port city of Mariupol, part of an anticipated massive onslaught across eastern Ukraine.
Moscow is believed to be trying to connect occupied Crimea with Russian-backed separatist territories Donetsk and Lugansk in Donbas, and has laid siege to the strategically located city, once home to more than 400,000 people.
Civilians were struggling to flee targeted zones, with Ukrainian President Volodymyr Zelensky condemning alleged mass rapes in areas previously occupied by Russian troops, including sexual assaults of small children.
As the fighting dragged toward its seventh week, the Ukrainian army fought desperately to defend Mariupol against the Russian offensive.
With little hope of a quick end to fighting, Putin pledged Moscow would proceed on its own timetable with its military operation, rebuffing repeated international calls for a ceasefire.
"Our task is to fulfill and achieve all the goals set, minimizing losses. And we will act rhythmically, calmly, according to the plan originally proposed by the General Staff," Putin said during a televised press conference with Belarusian President Alexander Lukashenko.
He also dismissed as "fake" reports of the discovery of hundreds of dead bodies of civilians in the town of Bucha outside the Ukrainian capital Kyiv after the withdrawal of Moscow's forces.
Meanwhile the mayor of the Ukrainian city of Mariupol on Tuesday said the latest estimate was that about 21,000 civilian residents of the port city had been killed since the start of Russia's invasion.
In televised comments, Mariupol Mayor Vadym Boichenko said it had been difficult to calculate the exact number of casualties since street fighting had started.
Ukraine's border force said Tuesday that more than 870,000 people who fled abroad since the start of the war had returned to the country, including a growing number of women and children.
'Helping people'
However, heavy bombardment continued in the east as civilians were urged to flee ahead of an expected Russian troop surge in the region.
Russian forces are reinforcing around the Donbas region, notably near the town of Izyum, but have not yet launched a full offensive, US Pentagon officials said on Monday.
They reported a Russian convoy had been observed heading for Izyum, an hour's drive north of Kramatorsk, saying it appeared to be a mix of personnel-carriers, armoured vehicles and possible artillery.
Putin insisted that Russia's own security was at stake in Donbas.
"What we are doing is helping people -- rescuing them on the one hand and taking measures to assure Russia's security," he said.
Putin accuses Ukraine of "inconsistency on fundamental points" which he said was slowing down talks on ending the war.
Kyiv admitted that ongoing talks with Russia to end the war were "extremely difficult."
"The Russian side adheres to its traditional tactics of public pressure on the negotiation process, including through certain public statements," Ukrainian presidential adviser Mykhaylo Podolyak said in written comments to reporters.
Meanwhile, the toll on towns previously occupied by Russian forces during their month-long offensive to take Kyiv was still coming to light.
The UN Security Council -- which on Monday held a session on the plight of women and children in Ukraine -- will hold another meeting next week on the humanitarian situation there, in a bid to keep pressure on Russia despite its veto power over the body, diplomats said.
'Rape and sexual violence'
Officials called for a probe into assaults against women during the conflict.
"We are increasingly hearing of rape and sexual violence," Sima Bahous, director of the UN women's agency, told the Council. "These allegations must be independently investigated to ensure justice and accountability."
Zelensky on Tuesday voiced anger about the repeated accounts of sexual violence against Ukrainians.
More than 4.6 million Ukrainian refugees have now fled their country, the United Nations refugee agency said -- 90% of them women and children.
The war has displaced more than 10 million people overall.
Chemical weapons allegations
Late Monday, Britain said it was trying to verify reports that Russia had also used chemical weapons in Mariupol.
Ukrainian lawmaker Ivanna Klympush said Russia had used an "unknown substance" and that people were suffering from respiratory failure.
But deputy defence minister Ganna Maliar said the purported chemical attack was more likely phosphorus munitions.
"Officials conclusions will be made later," she told Ukrainian television.
