The U.S. plans to unveil additional sanctions on Russia this week, President Biden’s national security adviser said, condemning reports of atrocities in parts of Ukraine while declining to label Russian forces’ alleged actions as genocide.

“The next stage of this conflict may very well be protracted,” Biden adviser Jake Sullivan said. “We should be under no illusions that Russia will adjust its tactics, which have included and will likely continue to include wanton and brazen attacks on civilian targets.”

Mr. Sullivan called reports of mass killings in the commuter town of Bucha tragic and shocking. He said the U.S. is weighing possible options for a war-crimes trial, which Mr. Biden called for earlier Monday. He declined to say whether such a trial should take place in the International Criminal Court or another venue.

Pressed by reporters about whether the U.S. had seen evidence of genocide in Ukraine, Mr. Sullivan said: “We have seen atrocities, we have seen war crimes, we have not yet seen a level of systematic deprivation of life of the Ukrainian people to rise to the level of genocide.” He said the U.S. would continue to monitor the issue.

Mr. Sullivan declined to detail any additional sanctions, but he said energy is one of the issues being discussed with European allies.

Mr. Sullivan said the U.S. believes Russia is revising its war plan to focus its offensive on eastern and parts of southern Ukraine. He also said Russian forces are redeploying from northern Ukraine to the Donbas region. He predicted that Russia would likely deploy dozens of additional battalion tactical groups consisting of tens of thousands of soldiers to the eastern part of Ukraine.

Russia could also seek to extend its presence in Ukraine beyond the Luhansk and Donetsk provinces, Mr. Sullivan said. Moscow will likely try to hold Kherson and keep up its attacks on Kharkiv, as well as continue to launch air and missile strikes across the country, including in the country’s capital, Kyiv, he said.