• Sullivan said he was briefed on the situation Tuesday morning when he spoke with his counterpart, Ukrainian chief of staff Andriy Yermak.
• More than 440 bodies, including civilians and children, have been discovered in Izium so far.
• Sullivan said the United States and international allies will "hold Russia accountable for this."

White House National Security Advisor Jake Sullivan said his Ukranian counterparts tell him the mass graves discovered in Izium, Ukraine, after Russian forces were pushed out, are in some ways "worse" than those discovered in Bucha in April.

Sullivan said he was briefed on the situation Tuesday morning when he spoke with Ukrainian chief of staff Andriy Yermak.

"He gave me a report about what the Ukrainians were discovering around Izium, and he put it quite bluntly: He said that this is in some ways worse than Bucha, and we will see more of these as we go, as Ukraine de-occupies towns that have previously been occupied by Russian forces. We are finding increasing evidence of these mass atrocities."

More than 440 bodies, including those of civilians and children, have been found in Izium since Russian troops withdrew from the city earlier this month. Police found a mass grave Friday with the bodies of 17 Ukrainian soldiers. Many had broken bones, hands tied behind their backs and other signs of torture.

"We are going to work with the international community through every mechanism that is available to hold Russia accountable for this," Sullivan said at Tuesday's press briefing.

Ukrainian President Volodymyr Zelenskyy said in a video address Saturday investigators had discovered more than 10 torture chambers in the liberated areas of Kharkiv region.

The bodies of 458 people were found after Russians left Bucha. The mayor of Mariupol said thousands of bodies were discovered in a mass grave discovered after Russian soldiers withdrew from that city in April.

The Russian Embassy in Washington, D.C., did not immediately respond to CNBC's request for comment.