US Attorney General Merrick Garland made a surprise visit to Ukraine on Friday and vowed to hold "Russian war criminals accountable" for their actions.
"We are here today in Ukraine to speak clearly, and with one voice: the perpetrators of those crimes will not get away with them," Garland said.
He went to Lviv in western Ukraine at the invitation of his Ukrainian counterpart to take part in the "United for Justice Conference."
Garland told the conference the United States stood beside Ukraine's war crimes investigators as they collect and catalogue evidence from blast sites that include hospitals, apartment buildings and schools, exhume mass graves and study human remains -- "in order to tell the stories of those who no longer can," according to a Justice Department transcript of his remarks.
Since the invasion began a year ago, Russia has been committing atrocities on the largest scale of any conflict since World War II, he said.
The United States has signed an agreement with Ukraine, Lithuania, Poland, Estonia, Latvia, Slovakia and Romania "that will strengthen our efforts to hold Russian war criminals accountable," he said.
The visit, Garland's second to Ukraine since the start of the conflict in February 2022, was not announced ahead of time for security reasons.
It came nearly two weeks after US President Joe Biden paid a visit to Ukraine and met with President Volodymyr Zelenskyy.
The United States is helping Ukraine investigate war crimes and Garland this week branded Yevgeny Prigozhin, head of Russia's mercenary Wagner military force, a war criminal.
In a statement issued on February 24 to mark the first anniversary of the Russian invasion, Garland said his department is "standing with our Ukrainian partners in pursuit of justice."
US Vice President Kamala Harris accused Russia last month of committing crimes against humanity in Ukraine, saying Moscow's forces had conducted "widespread and systemic" attacks on the country's civilian population.