Updated

The presidents of Poland, Lithuania, Latvia and Estonia visited Ukraine on Wednesday to meet President Volodymyr Zelenskyy.

The leaders traveled by train to meet Zelenskyy in the capital city of Kyiv as the Russians prepared for a major attack in the eastern Donbas region.

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Lithuanian President Gitanas Nauseda said they planned to deliver "a strong message of political support and military assistance."

Nauseda, Estonian President Alar Karis, Poland’s Andrzej Duda and Latvia's Egils Levits also planned to discuss investigations into alleged Russian war crimes, including the massacre of civilians.

Raisa Dovganuk remove debris from her apartment, heavily damaged after a Russian attack destroyed a building across the street, in Kharkiv, Ukraine, Tuesday, April 12, 2022. 

Raisa Dovganuk remove debris from her apartment, heavily damaged after a Russian attack destroyed a building across the street, in Kharkiv, Ukraine, Tuesday, April 12, 2022.  (AP Photo/Felipe Dana)

More than 720 people were killed in Kyiv suburbs and more than 200 were considered missing, according to the Interior Ministry. In Bucha alone, 403 bodies had been found.

Russian President Vladimir Putin has vowed to continue his assault on Ukraine until its "full completion." Russia said more than 1,000 Ukrainian troops had surrendered in besieged Mariupol – though the veracity of that claim could not be verified. 

Putin has also denied that his troops committed atrocities, repeatedly insisting that Russia "had no other choice" but to invade, that the offensive aimed to protect people in parts of eastern Ukraine and was to "ensure Russia’s own security." 

destroyed bridge Ukraine

A destroyed car and bridge in the village of Dymer, in Kyiv region, Ukraine, Tuesday, April 12, 2022. (AP Photo/Efrem Lukatsky)

He also said the war was going as planned – despite Russian forces suffering major losses and withdrawing from Kyiv. 

Britain’s defense ministry reported Wednesday that "an inability to cohere and coordinate military activity has hampered Russia’s invasion to date." 

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Western officials say Russia recently appointed a new top general, Alexander Dvornikov, to try to get a grip on its campaign.

Bucha civilians killed by Russians

Volunteers load bodies of civilians killed in Bucha onto a truck for transport to a morgue for investigation, in the outskirts of Kyiv, Ukraine, Tuesday, April 12, 2022. (AP Photo/Rodrigo Abd)

Mariupol – where food and ammunition are scarce – remains a key part of that plan, and Ukrainian presidential adviser Mykhailo Podoliak tweeted that the city's defenders were "fighting under the bombs for each meter of the city."

Ukraine is also investigating a claim that a drone dropped a poisonous substance on the city. 

The assertion by the Azov Regiment, a far-right group now part of the Ukrainian military, could not be independently verified. The regiment indicated there were no serious injuries.

Ukrainian Deputy Defense Minister Hanna Maliar said it was possible phosphorus munitions had been used there; they cause horrific burns but are not classed as chemical weapons.

school in Yahidne, near of Dnipro, Ukraine

Ammunition lies near a school in Yahidne, near of Dnipro, Ukraine, Tuesday, April 12, 2022. (AP Photo/Evgeniy Maloletka)

Western officials warned that any use of chemical weapons by Russia would be a serious escalation, and Zelenskyy called for the world to "react now." 

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President Biden called Russia's actions in Ukraine "a genocide" for the first time.

A senior U.S. defense official said the Biden administration was preparing an additional package of military aid for Ukraine to be announced, possibly totaling $750 million.

The Associated Press contributed to this report.