Biden told reporters a day earlier that he believed Russian President Vladimir Putin was a "war criminal" but the White House later said no formal determination had been made on potential violations of international law since Russia's Feb. 24 invasion of Ukraine.
"Personally, I agree," Blinken said in a news conference at the State Department on Thursday.
"Intentionally targeting civilians is a war crime," Blinken said, adding he found it "difficult to conclude that the Russians are doing otherwise" after the destruction during the past few weeks.
The Kremlin called Biden's comment "unacceptable and unforgivable rhetoric".Â
Moscow says it is conducting a "special military operation" to disarm and "denazify" its neighbour and has rejected claims of targeting civilians.
Blinken recounted recent incidents of Russia bombing hospitals, schools and a theatre where civilians were sheltering, and said they were part of "a long list of attacks on civilian, not military, locations across Ukraine".
Washington's new Ambassador at Large for Global Criminal Justice Beth Van Schaack, who was confirmed by the US Senate on Tuesday, would lead State Department efforts aimed at "documenting and evaluating potential war crimes being committed in Ukraine", Blinken said.
"We'll make sure that our findings help international efforts to investigate war crimes and hold those responsible accountable," Blinken said.
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