More disturbing photos are surfacing showing the massacre of civilians in and around Bucha, outside the capital of Kyiv, Ukraine, as Russian forces are reportedly regrouping in the southeast to renew their offensive against the already badly bombarded Mariupol, as well as the breakaway Donetsk and Luhansk regions. 

In an overnight address, Ukrainian President Volodymyr Zelenskyy said more than 300 people had been tortured and killed in Bucha, as satellite imagery seemed to back up Ukrainian claims about Russian soldiers shooting and killing civilians left lying in the streets and outside homes. 

The Kremlin has claimed that the bodies were staged after Russian soldiers pulled back from the capital region, allegations Ukraine and the West say is misinformation meant to displace blame for such war crimes. 

LIVE UPDATES: ZELENSKYY TO ADDRESS UN, GERMANY AND FRANCE EXPEL RUSSIAN DIPLOMATS IN BUCHA FALLOUT 

Russian forces on Tuesday were preparing for an offensive in Ukraine’s southeast, the Ukrainian military said, as Zelenskyy prepared to talk to the U.N. Security Council amid outrage over evidence Moscow’s soldiers deliberately killed civilians.

Russian President Vladimir Putin’s government is pouring soldiers into Ukraine’s east to gain control of the industrial heartland known as the Donbas. That follows a Russian withdrawal from towns around the capital, Kyiv, which led to the discovery of corpses and prompted accusations of war crimes and demands for tougher sanctions on Moscow.

WARNING: GRAPHIC IMAGES

Kremlin spokesman Dmitry Peskov said Tuesday that Russia is not opposed to a meeting between Putin and Zelenskyy, but stressed Moscow would only attend if a document is agreed on beforehand, according to Reuters. Russian news agency Interfax said discussions between Ukrainian and Russian officials are continuing via video link Tuesday, citing a deputy Russian foreign minister. 

Russian forces are focused on seizing the cities of Popasna and Rubizhne in the Donetsk and Luhansk regions, and the Black Sea port of Mariupol, Ukraine's General Staff said on its Facebook page. Donetsk and Luhansk are controlled by Russian-backed separatists and recognized by Moscow as independent states. The General Staff said access to Kharkiv in the east, Ukraine’s second-largest city, was blocked.

"The enemy is regrouping troops and concentrating its efforts on preparing an offensive operation in the east of our country," the statement said. "The goal is to establish full control over the territory of Donetsk and Luhansk regions."

Zelenskyy, speaking from Ukraine, planned to address U.N. Security Council diplomats Tuesday amid demands for an investigation of possible war crimes.

UKRAINE MAYOR, FAMILY KIDNAPPED BY RUSSIAN FORCES FOUND DEAD, TORTURED IN SHALLOW GRAVE: UKRAINIAN OFFICIALS 

Germany and France reacted by expelling dozens of Russian diplomats, suggesting they were spies. President Biden said Putin should be tried for war crimes.

Before Zelenskyy speaks, the most powerful U.N. body is due to be briefed by Secretary-General Antonio Guterres; his political chief, Rosemary DiCarlo; and U.N. humanitarian chief Martin Griffiths, who is trying to arrange a cease-fire. Griffiths met with Russian officials in Moscow on Monday and is due to visit Ukraine.

Associated Press journalists in Bucha counted dozens of corpses in civilian clothes and apparently without weapons, many shot at close range, and some with their hands bound or their flesh burned.

WARNING: GRAPHIC IMAGES 

After touring neighborhoods of Bucha and speaking to hungry survivors lining up for bread, Zelenskyy pledged in a video address that Ukraine would work with the European Union and the International Criminal Court to identify Russian fighters involved in any atrocities.

"The time will come when every Russian will learn the whole truth about who among their fellow citizens killed, who gave orders, who turned a blind eye to the murders," he said.

Among the dead was the mayor of the village of Motyzhyn, Olga Sukhenko, her husband and her adult son, whose tortured bodies were found in a shallow grave on a plot of land near houses used as makeshift barracks by occupying Russian troops, according to Ukrainian officials. The three of them reportedly had been kidnaped by Russian forces days before. 

Russian Foreign Minister Sergey Lavrov dismissed the scenes outside Kyiv as a "stage-managed anti-Russian provocation." Kremlin spokesperson Dmitry Peskov said the images contained "signs of video forgery and various fakes." Russia has rejected previous allegations of atrocities as fabrications by Ukraine.

In an initial count Sunday, Ukrainian prosecutor-general’s office said the bodies of at least 410 civilians have been found in towns around Kyiv that were recaptured from Russian forces, describing one room discovered in Bucha as a "torture chamber." In a statement, it said the bodies of five men with their hands bound were found in the basement of a children’s sanatorium where civilians were tortured and killed.

The bodies seen by AP journalists in Bucha included at least 13 in and around a building that local people said Russian troops used as a base. Three other bodies were found in a stairwell, and a group of six were burned together.

The dead witnessed by the news agency's journalists also included bodies wrapped in black plastic, piled on one end of a mass grave in a Bucha churchyard. Many of those victims had been shot in cars or killed in explosions trying to flee the city.

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About two-thirds of the Russian troops around Kyiv have left and are either in Belarus or on their way there, probably getting more supplies and reinforcements, said a senior U.S. defense official who spoke on condition of anonymity to discuss an intelligence assessment.

More than 1,500 civilians were able to escape Mariupol on Monday, using the dwindling number of private vehicles available to leave, Ukrainian Deputy Prime Minister Iryna Vereshchuk said. The besieged southern port city has seen some of the heaviest fighting of the war.

But amid the fighting, a Red Cross-accompanied convoy of buses that has been thwarted for days on end in a bid to deliver supplies and evacuate residents was again unable to get inside the city, Vereshchuk said.

The Associated Press contributed to this report.