A view shows a residential house destroyed by shelling, amid Russia’s invasion of Ukraine, in Derhachi, Ukraine March 21, 2022. REUTERS/Oleksandr Lapshyn

The United States has assessed that members of Russia’s forces have committed war crimes in Ukraine, US Secretary of State Antony Blinken said on Wednesday, adding that Washington’s conclusion was based on a “careful review” of available information from public and intelligence sources.

Blinken said there had been “numerous credible reports of indiscriminate attacks and attacks deliberately targeting civilians, as well as other atrocities” by Russia’s forces in Ukraine, specifying attacks in the besieged city of Mariupol.

Russia has denied targeting civilians.

In a statement, Blinken said the United States will continue to track reports of war crimes and will share information it gathers with allies and international institutions. A court of law would be ultimately responsible in determining any alleged crime, he said.

“We are committed to pursuing accountability using every tool available, including criminal prosecutions,” Blinken said.

President Joe Biden last week said Russian President Vladimir Putin was “a war criminal” for attacking Ukraine, which Russia’s foreign ministry said was a statement “unworthy of a statesman of such high rank.”

Moscow has yet to capture any of Ukraine’s biggest cities following its invasion that began on Feb. 24, the largest assault on a European state since World War Two.

Putin calls his offensive a “special military operation” to demilitarize and “denazify” the country. The civilian casualties are thought to be in the thousands while the United Nations estimates more than 3.5 million people have fled.

Investigators from the International Criminal Court set off earlier this month to start looking into possible war crimes in Ukraine. Washington has said it welcomed the decision, although it has no cooperation duties since it is not a member of the court.

Beth Van Schaack, Ambassador-at-Large for Global Criminal Justice at the State Department, said Washington was looking at the broad range of activities that Russia’s forces are engaged in within Ukraine.

The destruction of a theater in Mariupol last week “appears to have been a direct attack upon a civilian (target),” she said.

“This was very clearly marked with the word ‘children’… It’s not a military objective,” she said at a briefing at the State Department. Russia has denied bombing the theater.

Van Schaack said evidence like signals intelligence and accounts from Russian insiders could be used by courts to show that civilians were intentionally targeted. Such evidence was being preserved for that purpose, she said.

Legal experts say a prosecution of Putin or other Russian leaders would face high hurdles and could take years.