The Pentagon on Monday accused Russian forces of committing war crimes in Ukraine.
"We certainly see clear evidence that Russian forces are committing war crimes, and we are helping with the collecting of evidence of that," Pentagon press secretary John Kirby told reporters in response to a question by VOA.
Kirby also accused Russian forces of carrying out "indiscriminate attacks on civilians" that were "intentional" in many cases.
He said the United States would wait for ongoing investigative processes and contribute to those investigations into Russian war crimes.
In Moscow, Russia's Foreign Ministry said Monday it had summoned U.S. Ambassador John Sullivan, citing "unacceptable statements" by U.S. President Joe Biden about Russian President Vladimir Putin, an apparent reference to Biden calling the Russian leader a "war criminal" last week.
"Such statements from the American president, unworthy of a statesman of such high rank, put Russian-American relations on the verge of rupture," it said.
The Pentagon's accusation came as Biden warned there was "evolving intelligence" that the Russian government was "exploring options for potential cyberattacks" against the U.S. amid its multifront war against Ukraine.
In a statement on Monday, the president urged the private sector to "immediately" harden "cyber defenses."
He said Russia could launch such attacks as retaliation for U.S. sanctions on Russia.
"It's part of Russia's playbook," he said.
The leaders of the U.S., France, Germany, Italy and Britain held a call Monday in which they discussed concerns about Russia's brutal tactics in Ukraine, underscored their continued support to Ukraine by providing security and humanitarian assistance, and reviewed recent diplomatic efforts in support of Ukraine's effort to reach a cease-fire, according to the White House.
Later this week, Biden will attend a NATO summit, a G-7 meeting, and a European Council summit in Brussels, all focused on the situation in Ukraine, before traveling to Poland.
In its latest intelligence assessment, Britain's Defense Ministry said Monday that Russia's claims that it used hypersonic missiles against targets in western Ukraine are "likely intended to detract from a lack of progress in Russia's ground campaign."
It also said that use of such missiles is "highly unlikely to materially affect the outcome of Russia's campaign in Ukraine."
Ukraine's allies are continuing to supply Ukrainian fighters through ground shipments of weapons that have not been attacked, a senior U.S. defense official said Monday. Ukrainians still have more than 90% of their combat power after nearly four weeks of fighting, the official said, in part because the U.S. and other allies have replenished them "in real time."
Inside Ukraine, President Volodymyr Zelenskyy on Monday said Ukrainians would need to vote in a referendum on any compromises agreed with Russia to end the war.
"The people will have to speak up and respond to this or that form of compromise," he said in an interview published by Ukrainian public broadcasting company Suspilne.
A day earlier, Zelenskyy told CNN, "Russians have killed our children. You cannot reverse the situation anymore. You cannot demand from Ukraine to recognize some territories as independent republics. These compromises are simply wrong."
Kremlin spokesman Peskov told reporters Monday that more progress needs to be made before Zelenskyy and Putin could meet to commit to any agreement. Russia's lead negotiator has said in recent days that the sides have moved closer to agreement on the issue of Ukraine dropping its bid to join NATO and adopting neutral status.
Ukraine rejected a Russian ultimatum to surrender the besieged city of Mariupol in the early hours before dawn Monday, as Russian forces carried out more shelling on Kyiv, the Ukrainian capital.
European Union foreign policy chief Josep Borrell described Russia's actions in Mariupol as "awful."
"What's happening in Mariupol is a massive war crime. Destroying everything, bombarding and killing everybody in an indiscriminate manner," Borrell said as EU foreign ministers gathered in Brussels.
Shelling hit a shopping center in Kyiv overnight, killing at least eight people, while Kyiv Mayor Vitali Klitschko announced a new curfew until Wednesday morning.
He told The Washington Post on Monday that Russian attacks had destroyed 72 apartment buildings, preschools and other schools, injuring nearly 300 people.
A senior U.S. defense official said Russian forces are still about 15 kilometers northwest of Kyiv.
"They haven't achieved anything in terms of what we assessed to be their objectives, which was population centers, so that they could occupy and take over Ukraine," the official added, calling recent actions targeting civilians a "near-desperate attempt by the Russians to gain some momentum."
Millions of people have fled their homes since the invasion.
"The war in Ukraine is so devastating that 10 million have fled — either displaced inside the country or as refugees abroad," U.N. High Commissioner for Refugees Filippo Grande tweeted Sunday.
This week, the U.N. General Assembly is expected to resume an "emergency special session" to vote on a draft resolution prepared by France and Mexico demanding an immediate stop to Russia's hostilities against Ukraine, especially attacks on civilians and civilian infrastructure.
The text also demands the full protection of civilians — including humanitarians, medical personnel, journalists and foreign nationals — and people trying to flee the conflict.
In the besieged city of Mariupol, an art school where about 400 people had found shelter was bombed by Russian forces early Sunday.
Mariupol's city council said the building was destroyed in the attack. Information about survivors was not immediately available.
U.S. Defense Secretary Lloyd Austin told CBS' "Face the Nation" he thinks Russian forces are resorting to these brutal attacks on civilians because its military "campaign is stalled."
"This is really disgusting," Austin said.
Just a few days earlier, a Russian airstrike targeted a theater where hundreds of people had been sheltering. The word "CHILDREN" had been written in Russian in big letters outside the theater, visible from the sky and on the ground, to alert Russian forces of who was inside.
More than 100 have been rescued from the theater, and it is still unclear how many casualties the attack caused.
The city continues to resist Russian military forces, who are having to engage in attrition tactics and urban fighting that requires going from building to building.
Pentagon correspondent Carla Babb and U.N. correspondent Margaret Besheer contributed to this report.
Some information also came from The Associated Press, Agence France-Presse and Reuters.