(CNN) President Joe Biden on Monday said the atrocities allegedly committed by Russian forces in Bucha, Ukraine, are a "war crime" and called for a trial to take place against Russian President Vladimir Putin.
The US President did not, however, label the killings a "genocide" but said he was looking into additional sanctions against Russia.
Biden said the images coming from Bucha warranted calling Putin a "war criminal," adding, "but we have to gather the information. We have to continue to provide Ukraine with the weapons they need to continue the fight and we have to get all the details so this can be an actual -- have a war crime trial."
White House national security adviser Jake Sullivan said later Monday that the US hasn't yet seen evidence of "systematic" killings that would warrant designating what's underway in Ukraine a genocide.
"Based on what we have seen so far -- we have seen atrocities. We have seen war crimes. We have not seen a level of systemic deprivation of life of the Ukrainian people to rise to the level of genocide," Sullivan said during a White House press briefing.
But the images from Bucha, Sullivan said, underscore that "now is not the time for complacency," stressing the importance of ongoing US support for Ukraine. He said the administration is "working around the clock" to fulfill security assistance requests from Ukraine, detailing the US and allied response so far and hinting at forthcoming "additional military assistance in the coming days."
"We expect additional new capabilities to be delivered in the near future. We can't always advertise what is being delivered out of deference to our allies and partners or for operational sensitivities, but we are moving with speed and efficiency to deliver," he added.
Sullivan also confirmed that the US will issue additional sanctions against Russia this week, saying that the administration is "coordinating with our allies and partners on what the exact parameters of that will be."
The White House has declined to go into specifics of how Biden's call for a war crimes trial for Putin would move forward, with Sullivan saying the US would have to "consult with our allies and partners on what makes most sense as a mechanism."
Biden's assessment that the killings did not amount to a genocide put him at odds with that of Ukrainian President Volodymyr Zelensky, who used the term during an interview with CBS on Sunday.
"This guy (Putin) is brutal and what's happening in Bucha is outrageous and everyone's seen it," Biden said.
Images released this weekend show civilian bodies strewn across a street following the withdrawal of Russian forces, and CNN reporters observed a mass grave in the town, with residents saying they believe at least 150 people are buried there.
The scenes out of the Kyiv suburb of Bucha have drawn international outrage, with Western leaders calling for war crimes investigations and fresh sanctions against Russia.
US Ambassador to the United Nations Linda Thomas-Greenfield is expected to call on the UN to suspend Russia from the Human Rights Council on Tuesday, which she'll be doing at the President's direction, White House press secretary Jen Psaki said during Monday's press briefing.
"He believes it's ludicrous for Russia to be a member of the Human Rights Council and certainly the ambassador spoke to this today ... and she will continue to make the case in her role when she returns to New York," Psaki said.
US Secretary of State Antony Blinken said Sunday that the State Department would help document any attacks by Russian troops against Ukrainian civilians. NATO Secretary General Jens Stoltenberg called the deaths of civilians in Bucha a "brutality" and said "I strongly welcome" an investigation by International Criminal Court, which has opened an investigation into war crimes in Ukraine.
The Russian Ministry of Defense claimed the extensive footage of dead civilians in Bucha was "fake" and that "not a single local resident suffered from any violent actions" during Russia's occupation of the town. "In the settlements of the Kiev region, Russian military personnel delivered and issued 452 tons of humanitarian aid to civilians," the ministry said in a statement.
The Russian military, Sullivan said during the briefing, is "revising its war aims" in Ukraine, warning that the next phase of invasion could be "protracted" with Russia's aim being to "surround and overwhelm" east Ukraine.
Sullivan said the White House has assessed that Russia will focus on defeating forces in Luhansk and Donetsk provinces and could potentially extend its force projection and presence deeper into Ukraine.
This story has been updated with comments from national security adviser Jake Sullivan and White House press secretary Jen Psaki.