The US has formally concluded that Russia has committed crimes against humanity in Ukraine, vice-president Kamala Harris said on Saturday, vowing that those who had perpetrated crimes and their superiors “will be held to account”.

Speaking at the Munich Security Conference, Harris said that Washington had “examined the evidence, we know the legal standards, and there is no doubt: these are crimes against humanity”.

She added that Russia had been “weakened” by the war. “The transatlantic alliance is stronger than ever. And most importantly, the spirit of the Ukrainian people endures,” she said.

Also speaking in Munich, UK prime minister Rishi Sunak said that “winning the peace” would require rebuilding “the international order”, which would involve holding Russia to account for its actions.

“We must see justice through the [International Criminal Court] for their sickening war crimes committed, whether in Bucha, Irpin, Mariupol or beyond,” he said.

The “treaties and agreements of the post-Cold War era have failed Ukraine”, Sunak said.

The US has traditionally been reluctant to join international courts such as the ICC in The Hague, which launched an investigation into Russian war crimes in Ukraine in March last year, shortly after the war began.

Washington is concerned that by joining it would open itself up to international prosecution for its own past actions in wars such as Afghanistan and Iraq.

Ukraine and a number of European countries including the UK are exploring the possibility of setting up a special war tribunal to hold Russia to account.

London is set to host an international conference on the subject next month.

Harris’s and Sunak’s remarks came hours after US secretary of state Antony Blinken spoke of Washington’s “deep concern” that Moscow was working to destabilise Moldova and overthrow the government of the eastern European nation.

Speaking after a meeting with Moldovan president Maia Sandu in Munich, Blinken said the US was alarmed by “some of the plotting that we’ve seen coming from Russia to try to destabilise the government”.

Blinken added that the US would continue to “stand strongly with Moldova in support of its security, its independence, its territorial integrity”.

Moldovan and Ukrainian officials have claimed in recent weeks that the eastern European nation, which borders Ukraine, was being targeted by a Russia plot. Blinken’s comments make him the most prominent western official to publicly make the same accusation.

Speaking at the same event, Sandu said Russia was waging “hybrid warfare” against her country using factors such as economic pressure and propaganda.

The aim was to “destabilise society” and “bring people into the streets to change the government and create a pro-Russian government”, she said.

Sandu acknowledged there was no social consensus in Moldova on whether to apply for Nato membership, which she blamed on Russian propaganda. She asked for help on air surveillance and defence, but said she saw no imminent threat of invasion.

Last week Ukrainian president Volodymyr Zelenskyy told EU leaders in Brussels that his country has intercepted plans by Russian intelligence to “break the democracy of Moldova and establish control over Moldova”.

Moldova’s intelligence service subsequently said that it had identified “subversive activities, aimed to undermine the Republic of Moldova, destabilise and violate public order”.

Russia has denied those claims. Foreign Ministry spokeswoman Maria Zakharova said last week that the allegations were “absolutely unfounded and unsubstantiated”.

The threat posed to European security by Russian aggression has dominated the Munich Security Conference, where senior foreign policy, defence and intelligence officials from across the west have gathered for three days of meetings on the eve of the first anniversary of the Russian invasion of Ukraine.

Moldova, a nation of less than 3mn people, has been reeling from the effects of the conflict in neighbouring Ukraine with an influx of refugees, the severing of gas and electricity supplies and a collapse in foreign trade.

Speaking after a meeting with US president Joe Biden, president Sandu said that 2022 had been an “incredibly difficult year for Moldova” but thanked the US for its “comprehensive assistance”.

Blinken also discussed the situation in the country with German foreign minister Annalena Baerbock on the sidelines of the summit. A person familiar with the discussion said that the two officials agreed that Russia was seeking to destabilise the eastern European nation.

Jens Stoltenberg, Nato secretary-general, told the conference that Putin “is planning for more war, new offensives”, citing Russia “mobilising hundreds of thousands of troops, increasingly putting the Russian economy on a war footing, and reaching out to other authoritarian regimes, such as Iran and North Korea, to get more weapons”.

Stoltenberg also warned that “Beijing is watching closely to see the price Russia pays or the reward it receives for its aggression”.

“What is happening in Europe today could happen in east Asia tomorrow. The war in Ukraine demonstrates that security is not regional, it is global,” Stoltenberg said.

Additional reporting by Jasmine Cameron-Chileshe, Henry Foy, Gideon Rachman and Reuters