Russian forces on Tuesday were preparing for an offensive in Ukraine’s southeast, the Ukrainian military said, as President Volodymyr Zelenskyy prepared to talk to the United Nations Security Council amid outrage over evidence of deliberate killings of civilians.

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

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, the 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," it added.

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

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

"This guy is brutal, and what’s happening in Bucha is outrageous," Biden said, referring to the town northwest of the capital that was the scene of some of the horrors.

Before Zelenskyy speaks, the most powerful U.N. body was 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 (AP) 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.

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 (ICC) 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.

'Bucha allegations are propaganda'

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.

"These are fakes that matured in the cynical imagination of Ukrainian propaganda," Dmitry Medvedev, who served as president from 2008 to 2012 and is now deputy secretary of Russia's Security Council, said on Telegram.

Russia's Defense Ministry said it had evidence that the 72nd Ukrainian Main Center for Psychological Operations had helped stage such propaganda in a village 23 kilometers (14 miles) northwest of Kyiv as well as in Sumy, Konotop and other towns.

"Soldiers of the 72nd Ukrainian Main Center for Psychological Operations conducted another staged filming of civilians allegedly killed by the violent actions of the Russian armed forces," the ministry said. It gave no further details.

Moscow has questioned why, if its forces withdrew from Bucha on March 30 and the mayor of Bucha declared the area free of Russian forces on March 31, that the bodies of dead civilians were only shown for the first time on April 3.

Russia also says the bodies shown in some footage did not show characteristic signs of degradation that would be expected after a number of days.

Russia's Medvedev said the fakes had been concocted for vast amounts of money by Western public relations companies and "tame" nongovernmental organizations (NGOs). He did not provide specific evidence. He suggested that Ukrainian forces had been prepared to kill their own citizens in a bid to discredit Russia.

"To dehumanize Russia and for her maximum vilification, the frenzied beasts from the national battalions and territorial defense units are prepared to kill even their own civilians," Medvedev said.

Ukrainian officials said the bodies of at least 410 civilians have been found in towns around Kyiv that were recaptured from Russian forces.

The Ukrainian prosecutor-general’s office described 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. With the morgue full and the cemetery impossible to reach, the churchyard was the only place to keep the dead, Father Andrii Galavin said.

Tanya Nedashkivska said she buried her husband in a garden outside their apartment building after he was detained by Russian troops. His body was one of those left heaped in a stairwell.

"Please, I am begging you, do something!" she said. "It’s me talking, a Ukrainian woman, a Ukrainian woman, a mother of two kids and one grandchild. For all the wives and mothers, make peace on Earth so no one ever grieves again," she pleaded.

Another Bucha resident, Volodymyr Pilhutskyi, said his neighbor Pavlo Vlasenko was taken away by Russian soldiers because the military-style pants he was wearing and the uniforms that Vlasenko said belonged to his security guard son appeared suspicious. When Vlasenko’s body was later found, it had burn marks from a flamethrower, his neighbor said.

Russia’s U.N. Ambassador Vassily Nebenzia insisted Monday at a news conference that during the time that Bucha was under Russian control, "not a single local person has suffered from any violent action."

However, high-resolution satellite imagery by commercial provider Maxar Technologies showed that many of the bodies have been lying in the open for weeks, during the time that Russian forces were in Bucha. The New York Times first reported on the satellite images showing the dead.

Western and Ukrainian leaders have accused Russia of war crimes before. The ICC’s prosecutor has already opened an investigation. But the latest reports ratcheted up the condemnation.

German Foreign Minister Annalena Baerbock said the images from Bucha reveal the "unbelievable brutality of the Russian leadership and those who follow its propaganda."

French President Emmanuel Macron said there is "clear evidence of war crimes" in Bucha that demand new punitive measures.

"I’m in favor of a new round of sanctions and in particular on coal and gasoline. We need to act," he said on France-Inter radio.

Though united in outrage, the European allies appeared split on how to respond. While Poland urged Europe to quickly wean itself off Russian energy, Germany said it would stick with a gradual approach of phasing out coal and oil imports over the next several months.

Russia withdrew many of its forces from the area around Kyiv after being thwarted in its bid to swiftly capture the capital. It has instead poured troops into southeastern Ukraine.

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.

Elsewhere, Russian shelling killed 11 people in the southern city of Mykolaiv, regional governor Vitaliy Kim said in a video message on social media.

Zelenskyy appealed for more weaponry as Russia prepares new offensives.

"If we had already got what we needed — all these planes, tanks, artillery, anti-missile and anti-ship weapons – we could have saved thousands of people," he said.

EU leaders head to Ukraine

European Commission President Ursula von der Leyen will travel "this week" to Kyiv, accompanied by EU diplomatic chief Josep Borrell, her spokesperson said on Tuesday.

The pair, two of the European Union's most senior figures, "will travel to Kyiv this week to meet with President Zelenskyy ahead of the #StandUpForUkraine event in Warsaw on Saturday," commission spokesperson Eric Mamer said in a tweet.

Zelenskyy told reporters in Ukraine that "I expect her visit in the coming days, we don't yet know the timing."

He said he and von der Leyen had agreed on Sunday that a joint Ukraine-EU investigation body would probe possible war crimes that Kyiv blames on Moscow's troops. Von der Leyen on Monday said the EU was "ready to reinforce this effort" by sending investigation teams to Ukraine.

The EU is discussing a fifth round of sanctions on Russia for its invasion of Ukraine, with member states expected to sign off on it this week or next.

The EU chief's trip to Ukraine was revealed ahead of its official announcement by Slovenian Prime Minister Janez Jansa on Monday. Her visit is to come after one made last Friday by the speaker of the European Parliament, Roberta Metsola – the first trip to Ukraine by the head of an EU institution since the war began.

Jansa and his Polish and Czech counterparts on March 15 visited Kyiv in the first trip by EU leaders.

The "Stand Up For Ukraine" event in Warsaw referred to in the tweet by von der Leyen's spokesperson is the culmination of a global donation drive jointly organized by the EU and Canada to raise money for Ukrainian refugees.