“We are clear that any intentional attack or targeting of civilians is a war crime. Period,” Ms. Harris said during a news conference with Romania’s president.

Ms. Harris previously offered support for efforts to investigate whether Russia has committed war crimes in Ukraine, saying the U.S. would work with international efforts already under way. White House press secretary Jen Psaki said Thursday that the legal review process the U.S. uses to label acts as war crimes is proceeding, saying that the review was necessary to make a formal conclusion.

Ms. Harris also stressed the U.S. commitment to NATO allies and spoke about the refugee crisis triggered by the Russian invasion of Ukraine. “It is painful to watch what is happening to innocent people in Ukraine, who just want to live in their own country and have pride in themselves as Ukrainians,” she said.

She said the U.S. had been trying to engage in diplomacy from the outset: “From everything that we know and have witnessed, Putin shows no sign of engaging in serious diplomacy.”