"We have examined the evidence, we know the legal standards, and there is no doubt: these are crimes against humanity," said Harris at the high-level annual defence gathering.

"I say to all those who perpetrated these crimes and their superiors who are complicit in these crimes: You will be held to account."

Harris accused Russian forces of "widespread and systematic attack against a civilian population."

Specifically, she accused the troops of murder, torture, rape and deportation. Hundreds of thousands of people had been forcibly taken to Russia, including children, she said. "They cruelly separated children from their families," she said.

Crimes against humanity are serious violations of international law, which are carried out systemically to cause large-scale suffering to civilians.

Later at the conference, the Group of Seven (G7) industrialized nations warned third countries against providing military support to Russia for Moscow's war on Ukraine, in a joint statement released by foreign ministers at the Munich Security Conference.

The G7 is determined to sanction countries that provide material support for Russia's illegal war on Ukraine or circumvent sanctions on Russia. Such countries would face significant costs, the statement said.

The G7 includes Germany, the United States, France, Britain, Italy, Canada and Japan.

The statement came after Harris and US Secretary of State Antony Blinken warned others against providing military support to China.

US President Joe Biden also made it clear to Chinese President Xi Jinping that this would be considered a very serious problem, and was an issue that was being closely watched.

Meanwhile Russia is also paying a high price for the war, Blinken told the conference.

The United States calculates that some 200,000 Russians have already been killed or wounded in the invasion of Ukraine.

He said the cost of the almost year-long war ran still deeper for Russia. "We have more than a million Russians who have left their country because they do not want to be part of this war and the direction that the country is being taken."

Additionally, 1,000 or more companies stopped doing business in Russia for fear of reputational damage, according to the diplomat.

"Look at what has happened, what Putin has done to his own country," Blinken said, referring to Russian President Vladimir Putin.