Washington has seen "very credible reports" that Russia has committed war crimes during its invasion of Ukraine, particularly in attacking civilians, US Secretary of State Antony Blinken said Sunday.
But President Joe Biden's top diplomat also hailed the resilience of the Ukrainian people and said Russian leader Vladimir Putin was "destined to lose" the war that has already claimed hundreds of lives, including civilians.
"We've seen very credible reports of deliberate attacks on civilians, which would constitute a war crime," Blinken told CNN talk show "State of the Union."
Blinken was speaking from Moldova, where he was expected to provide reassurances for the small country, and where he earlier said the United States was "working actively" with Poland on a deal that would supply Ukraine with fighter jets to battle Moscow's invading forces.
Russia has come under intense criticism for its assault on Ukrainian cities, in operations that Kyiv and Western governments say have included attacks on schools, hospitals and residential blocks.
On Thursday, Putin's forces attacked the Zaporizhzhia nuclear power plant, Europe's largest, triggering fears of a catastrophic atomic accident.
The US embassy in Ukraine the next day called the attack attributed to Russian forces a possible war crime.
The State Department was more cautious, saying it was assessing the circumstances of the operation but adding that intentional targeting of civilians or civilian objects including nuclear power plants "is a war crime."
With Russia laying siege to several Ukrainian cities, Blinken on Sunday warned of a protracted war and "terrible" suffering.
"I think we have to be prepared for this to last for some time. But just winning a battle is not winning the war. Taking a city does not mean he's taking the hearts and minds of the Ukrainian people," Blinken said.