Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Another mass grave has been dug up in Bucha, uncovering the bodies of seven men who authorities believe were tortured and killed during the bloody occupation of the city in March.

Police told ABC News their hands were tied with ropes behind their backs and they were shot in the knees and head.

"They were killed in a cruel way," police spokesperson Iryna Pryanyshnykova said. "These were civilian victims. The people here were killed by Russian soldiers and later they were just put into a grave to try to hide this war crime."

It's not clear why the men were killed, Pryanyshnykova said.

She said experts will analyze DNA to identify the victims.

-ABC News' Britt Clennett

Russian forces have pushed the Armed Forces of Ukraine out of the center of Severodonetsk, Ukrainian officials said.

"They are pressing in Severodonetsk, where very fierce fighting is going on -- literally for every meter," Ukrainian President Volodymyr Zelenskyy said in an address on Sunday evening.

Russian forces now control about 70% of the city, as intense shelling makes mass evacuation and the transportation of goods impossible, Sergiy Haidai, another Ukrainian official, said.

Around 500 people, including 40 children, are sheltering in the city's Azot chemical plant, Haidai said.

While the Ukrainians try to organize their evacuation, authorities of the self-declared Donetsk People's Republic have given an ultimatum to Ukrainian troops in the city.

"They have two options: either follow the example of their colleagues and give up, or die. They have no other option," said Eduard Basurin, deputy head of the People's Militia Department of the DPR.

-ABC News' Yulia Drozd and Tanya Stukalova

Ukrainian President Volodymyr Zelenskyy addressed the annual Hollywood fundraiser for actor Sean Penn's nonprofit Community Organized Relief Effort (CORE) Saturday night with a powerful video message urging people to continue to support Ukraine in its war against Russia.

"All of you have heard about the horrors that Ukraine is going through. Tens of thousands of explosions and shots, hundreds of thousands wounded and killed, millions who have lost their homes," Zelenskyy said in his virtual speech. "All of this is not a logline for a horror film. All of this is our reality."

Zelenskyy's video message included footage showing missiles striking homes and apartment complexes in Ukraine, civilians dead in the streets of Ukrainian cities and children playing in parks amid the backdrop of bombed buildings.

Among those attending the CORE fundraiser, held at the Hollywood Palladium in Los Angles, were Penn and CORE co-founder Ann Lee, former President Bill Clinton, Los Angeles Mayor Eric Garcetti, singer John Legend, and actors Patrick Stewart and Sharon Stone.

The group said the event raised more than $2.5 million for CORE's disaster relief and preparedness work, including its urgent humanitarian response in Ukraine.

Zelenskyy noted that Penn traveled to Ukraine at the start of the Russian invasion and witnessed the atrocities firsthand. He thanked Penn and his group for the continued support for Ukraine.

"We have been resisting it for 107 days in a row," Zelenskyy said of Russia's aggression in Ukraine. "We can stop it together. Support Ukraine, because Ukraine is fighting for the whole world, for democracy, for freedom, for life."

Ukraine's Armed Forces Commander-in-Chief Valeriy Zaluzhny said Sunday that he told his American counterpart, Gen. Mark Milley, U.S. Chairman of the Joint Chiefs of Staff, that Russian firepower superiority in the Luhansk region is far greater than that of Ukrainian forces.

Zaluzhny said that during a briefing he told Milley that Russian forces are concentrating their efforts in the north of the Luhansk region, where they are using artillery "en masse" and their firepower superiority is 10 times that of Ukraine's.

"Despite everything, we keep holding our positions," Zaluzhny said.

Zaluzhny also said Russia has deployed up to seven battalion tactical groups in Severdonetsk, a city in the Luhansk region. He said Russian shelling of residential areas in Kharkiv in northeast Ukraine has resumed.

Russian forces destroyed a second bridge leading into Severodonetsk and are now targeting a third bridge in an effort to completely cut off the city, Luhansk region Gov. Sergiy Haidai said Sunday. Ukraine's army still controls around one third of the city, he said.

Haidai said that Ukrainian forces are still holding onto the Azot chemical plant in Severodonetsk, where around 500 civilians are taking shelter.

If Severodonetsk falls, Lysychansk will be the only city in the Luhansk region that remains under Ukraine's control.

Zaluzhny said that as of Sunday, the front line of the war stretched 1,522 miles and that active combat was taking place on at least 686 miles of the front line.

Zaluzhny said that during his briefing with Milley, he reiterated Ukraine's urgent request for more 155 mm caliber artillery systems.