Russia said Friday it intended a full-bore assault to carve out control of a sizeable slice of Ukraine but faced intensified world outrage as the UN documented the killing of dozens of civilians in one town.

Ukraine's government, emboldened by a fresh influx of high-grade Western weaponry, said its trapped forces were still holding out inside a sprawling, seaside steelworks in the razed city of Mariupol.

The Kremlin has claimed the "liberation" of Mariupol, whose control is pivotal to its war plans nearly two months after President Vladimir Putin ordered the shock invasion of Russia's Western-leaning neighbour.

"Since the start of the second phase of the special operation... one of the tasks of the Russian army is to establish full control over the Donbas and southern Ukraine," Major General Rustam Minnekaev said.

"This will provide a land corridor to Crimea," he added, according to Russian news agencies, referring to the peninsula Russia annexed from Ukraine in 2014.

Minnekaev's comments were the most detailed description yet of Russia's goals in the "second phase" of its operation, which was forced on the Kremlin after Ukraine's dogged resistance around the capital Kyiv.

Russia's change of strategic focus to southern and eastern Ukraine saw invading forces leave behind a trail of indiscriminate destruction and civilian bodies around Kyiv, including in the commuter town of Bucha.

A United Nations mission to Bucha documented "the unlawful killing, including by summary execution, of some 50 civilians there", the UN's Office of the High Commissioner for Human Rights said.

"Russian armed forces have indiscriminately shelled and bombed populated areas, killing civilians and wrecking hospitals, schools and other civilian infrastructure, actions that may amount to war crimes," its spokesperson Ravina Shamdasani added.

All being investigated

The UN mission was sent on April 9, a week after an AFP team found bodies of people dressed in civilian clothing lining the streets of Bucha, after the town had been under Russian occupation for over a month.

There was no immediate response from Russia, but it has previously accused Ukrainian forces of staging the scene in Bucha, which when first reported triggered Western denunciations and redoubled sanctions against Moscow.

US satellite imagery company Maxar released photos that it said showed a "mass grave" on the northwestern edge of Manhush, west of Mariupol.

UK police said they had received "around 50" referrals relating to alleged war crimes in Ukraine, in support of an investigation by the International Criminal Court at The Hague.

Ukrainian officials say the bodies of more than 1,000 civilians have been retrieved from areas around Kyiv, and they are working with French investigators to document alleged war crimes.

"It's all being investigated," Oleksandr Pavliuk, head of the Kyiv regional military administration told reporters. "There is no final number of civilians killed."

"The forensic experts are now examining the bodies, but what we saw was hands tied behind the back, their legs tied and shot through the limbs and in the back of the head," he said. 

Referendum for 'show

Ukrainian President Volodymyr Zelensky maintained a defiant tone in his latest video address.

With its claims about taking control of Mariupol, Russia was "doing everything to have a reason to talk about at least some victories", he said.

"They can only delay the inevitable -- the time when the invaders will have to leave our territory, in particular Mariupol, a city that continues to resist Russia, despite everything the occupiers say."

Zelensky accused Russia of laying the groundwork for a referendum to cement its control of separatist areas in the Donbas region of eastern Ukraine, urging locals to avoid giving personal data to Moscow's forces.

"This is aimed to falsify the so-called referendum on your land, if an order comes from Moscow to stage such a show," he warned.

Separately, Zelensky told leaders of the International Monetary Fund and World Bank that Ukraine now needs $7 billion a month to function, accusing Russia of "destroying all objects in Ukraine that can serve as an economic base for life".

He also said Russia had rejected a proposed truce over the Orthodox Christian Easter holiday this weekend, while the head of Ukraine's Moscow-affiliated branch of the Orthodox Church called for an Easter procession to the Azovstal steel plant in Mariupol.

On Thursday, three school buses carrying evacuees arrived in the city of Zaporizhzhia after leaving Mariupol and crossing through Russian-held territory.

"I don't want to hear any more bombing," said Tatiana Dorash, 34, who arrived with her six-year-old son Maxim.

She said all they wanted now was a quiet night and "a bed to sleep in".

Ukrainian officials had hoped to evacuate many more civilians, but called off evacuations on Friday because of "insecurity" on the roads.

"To all those who are waiting for an evacuation, please be patient and hold on," Deputy Prime Minister Iryna Vereshchuk said.

Critical window

Zelensky meanwhile welcomed the latest promises of Western military aid including, from the United States, howitzers, armoured vehicles, 144,000 rounds of ammunition and tactical drones.

"We're in a critical window now... where they're going to set the stage for the next phase of this war," US President Joe Biden said, pledging Putin would "never succeed in dominating and occupying all of Ukraine".

"That will not happen," he added, as he also announced a ban on Russian-affiliated ships using US ports.

The UK government said more than 20 Ukrainian soldiers had travelled to Britain for training in operating some 120 UK-supplied armoured vehicles.

Prime Minister Boris Johnson's spokesman denied that training the Ukrainians in Britain would risk escalating the conflict.

"We're always conscious of anything that's perceived to be escalatory, but clearly what's escalatory is the actions of Putin and his regime," he said.