Russia gave the most sombre assessment so far of its invasion of Ukraine, describing the "tragedy" of mounting troop losses and the economic hit from sanctions, as Ukrainians were evacuated from eastern cities before an anticipated major offensive.

Russia's six-week-long incursion has seen more than 4 million people flee abroad, killed or injured thousands, turned cities into rubble, and led to sweeping sanctions on its leaders and companies.

In a symbolic move, the U.N. General Assembly suspended Russia from the U.N. Human Rights Council, expressing "grave concern at the ongoing human rights and humanitarian crisis". Russia then quit the council.

Russia has previously acknowledged its attack has not progressed as quickly as it wanted but on Thursday Kremlin spokesperson Dmitry Peskov lamented the rising death toll.

"We have significant losses of troops," he told Sky News. "It's a huge tragedy for us."

Ukraine's military general staff said on Friday that Russian forces were focused on capturing the besieged southern port of Mariupol, fighting near the eastern city of Izyum and breakthroughs by Ukrainian forces near Donetsk.

Ukraine said it aimed to establish up to 10 humanitarian corridors on Friday, but civilians trying to flee Mariupol will have to use private vehicles.

Russia is facing its most difficult economic situation for three decades due to unprecedented Western sanctions, Prime Minister Mikhail Mishustin said. The U.S. Congress removed its "most favored nation" trade status in a further blow.

Russia says it launched what it calls a "special military operation" on Feb. 24 to demilitarise and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext.

Following new restrictions after the killing of civilians in the town of Bucha that were widely condemned by the West as war crimes, Ukraine called on allies to stop buying Russian oil and gas, amid divisions in Europe, and to boost it militarily.

"Ukraine needs weapons which will give it the means to win on the battlefield and that will be the strongest possible sanction against Russia," President Volodymyr Zelenskiy said in a video late on Thursday.

He also said the situation in Borodianka - another town northwest of Kyiv retaken from Russian forces - is "significantly more dreadful" than in Bucha, without citing any evidence.

Video from Borodianka showed search and rescue teams using heavy equipment to dig through the rubble of a building that collapsed. Hundreds of people were feared buried.

Russia has denied targeting civilians and says images of bodies in Bucha were staged to justify more sanctions against it and derail peace negotiations.

The EU's ambassadors agreed a fifth sanctions package on Russia with a coal embargo containing a 120-day wind-down period to give member states time to find alternative suppliers, following pressure from Germany to delay the measure.

Ukraine accused Hungary of undermining EU unity after Budapest said it was prepared to pay roubles for Russian gas, a Kremlin demand that most in the West had resisted.

On the battlefield, Ukraine says after withdrawing from Kyiv's outskirts, Russia is regrouping to try to gain full control of the eastern regions of Donetsk and Luhansk, which have been partly held by Russian-backed separatists since 2014.

The besieged southern port of Mariupol, where the mayor said over 100,000 people were still trapped, was also a target.

"Evacuate! The chances of saving yourself and your family from Russian death are dwindling every day," Luhansk governor Serhiy Gaidai said.

Authorities in Dnipro, a city in central-eastern Ukraine, also urged women, children, and the elderly to leave.

British military intelligence said Russian forces were shelling cities in the east and south and had advanced further south from the city of Izium, which is under their control. Reuters could not immediately verify the report.

TRADING ACCUSATIONS

Both sides have traded accusations of abuse, with Moscow opening a criminal investigation into a Russian soldier's allegations that he was beaten and threatened with death while being held in Ukraine as a prisoner of war.

Separately, a social media video verified by Reuters and geolocated to an area west of Kyiv appears to show Ukrainian forces shooting and killing a captured and badly wounded Russian soldier.

NATO members agreed to strengthen support to Ukraine on Thursday.

Ukraine has received about 25,000 anti-aircraft weapons systems from the United States and its allies, the top U.S. general said, and Washington is looking into what new support it could send.

Ukrainian Defence Minister Oleksii Reznikov made a plea for heavier, more modern weapons as Russia concentrated its forces for a major attack.

Russia has “drawn conclusions and changed tactics” and is now focusing on long-range strikes from the air, he said in a video address.

As a result, Ukraine needs air defence systems, long-range artillery, tanks and anti-ship missiles, Reznikov said.

Zelenskiy's government says starving Russia's war machine is the only way to bring it to a settlement at on-and-off peace talks.

On Thursday, Russian Foreign Minister Sergei Lavrov said a draft peace deal from Kyiv contained "unacceptable" elements and deviated from previously agreed proposals. read more

Ukrainian negotiator Mykhailo Podolyak told Reuters Lavrov was not directly involved in negotiations and his statements were "of purely propagandistic significance" aimed at diverting attention from the killings in Bucha.

Ukraine's prosecutor general, Iryna Venediktova, said in the Kyiv region, which includes Borodianka, Bucha and other towns and villages such as Irpin, authorities had found 650 bodies, with 40 of them children.

Ukraine's prosecutors said 169 children had been killed and 306 wounded in the invasion.

Bucha's mayor has said dozens were the victims of extra-judicial killings carried out by Russian troops. Reuters could not independently verify those figures.

Accounts by at least a dozen residents of one apartment complex in Bucha painted a picture of violence and intimidation by Russian soldiers.