US Vice President Kamala Harris on Saturday accused Russia of committing "crimes against humanity" in Ukraine, saying Moscow's forces had conducted "widespread and systemic" attacks on the country's civilian population.

She made the comments at the Munich Security Conference, days ahead of the one-year anniversary of Moscow sending its forces into Ukraine, unleashing war in Europe for the first time in decades.

"The US has formally determined that Russia has committed crimes against humanity," she said in an address to world leaders at the gathering.

It is the first time that the United States has formally designated Russia's actions in Ukraine as crimes against humanity.

"Their actions are an assault on our common values and our common humanity. Russian forces have pursued a widespread and systemic attack against a civilian population," Harris added.

She listed a litany of abuses she said were carried out by Moscow's forces in Ukraine -- "gruesome acts of murder, torture, rape and deportation, execution-style killings, beatings and electrocution".

"I say to all those who have perpetrated these crimes and to their superiors who are complicit in these crimes you will be held to account... Justice must be served," said Harris, a former prosecutor.

The three-day conference is being attended by world leaders, including those from France and Germany, and China's top diplomat Wang Yi.

Harris also hailed Ukraine's success in standing up to Russia's invasion. 

"Kyiv is still standing, Russia is weakened, the transatlantic alliance is stronger than ever and, most importantly, the spirit of the Ukrainian people endures," she said. 

'Massive support'

The second day of the conference on Saturday also heard calls for more military support for Ukraine.

Allies, led by the United States, have sent billions of dollars of armaments to Kyiv, from artillery to air defence systems, but Ukraine says it needs more to launch a successful counter-offensive.

"We must give Ukraine what they need to win and prevail as a sovereign, independent nation in Europe," NATO Secretary General Jens Stoltenberg said.

"The biggest risk of all is if Putin wins. If Putin wins in Ukraine, the message to him and other authoritarian leaders will be that they can use force to get what they want."

NATO Secretary General Jens Stoltenberg urged allies to 'give Ukraine what they need'. Photo: AFP

EU chief Ursula von der Leyen called for bolstered military support in areas such as ammunition supplies: "We have to double down, and we have to continue the really massive support that is necessary." 

Opening the conference on Friday via video link, Ukrainian President Volodymyr Zelensky urged allies to speed up their efforts.

Despite the calls for more backing, there is hesitancy on the part of allies to provide everything Ukraine wants.

While Berlin last month agreed German-made heavy battle tanks could be sent to Ukraine after weeks of hesitation, they have since struggled to get commitments from allies to join them in sending crucial armaments.

Zelensky's pleas for Western backers to give combat jets to Kyiv has received a cool response, with allies downplaying the prospect of that happening any time soon.

On the battlefield, Moscow on Friday claimed a small gain in its grinding offensive, with mercenary group Wagner reporting the capture of a village near Bakhmut -- the scene of the longest and bloodiest battles of the war.