There is mounting evidence of mass graves in the besieged city of Mariupol, UN officials said on Friday, as Russia hinted it was scaling back its invasion to focus on eastern Ukraine.

Matilda Bogner, head of a UN human rights team currently in the country, said monitors had received information about graves in the southeastern port city, including one that appeared to hold 200 bodies. The claims of mass graves in the city came as Ukraine said it feared around 300 people had been killed in the Mariupol theatre bombing on 16 March.

“We have got increasing information on mass graves that are there,” Ms Bogner said. “The extent of civilian casualties and the destruction of civilian objects strongly suggests that the principles of distinction, of proportionality, the rule on feasible precautions and the prohibition of indiscriminate attacks have been violated,” she added.

Meanwhile, in what is potentially a strategic turning point in the war, US officials say Russia may have halted, for now, its attempts to capture Kyiv, and is instead concentrating on fighting for control of the Donbas region in the southeast of the country, where Mariupol is situated.

This move also appeared to be confirmed by the Kremlin. A senior military official, Colonel General Sergei Rudskoi, deputy chief of the Russian general staff, said on Friday that the main objective of Moscow’s war had been “generally” accomplished, which would allow forces to focus on “the main goal, liberation of Donbas”.

The apparent shift in Moscow’s stated aims – after Vladimir Putin denied for weeks that Ukraine had the right to exist as a sovereign country – could point to a possible exit strategy for the Russian president.

But Russia’s armed forces – and the country’s leadership – still potentially face being charged with war crimes.

Ms Bogner’s UN team is investigating alleged human rights violations, including reports that Russian forces shot and killed civilians in their cars as they were fleeing Mariupol. Also under investigation are the disappearances of dozens of Ukrainian officials and journalists, and the forced movement of civilians into Russian-held territory.

Mariupol has become the grim symbol of an increasingly brutal war, and of Ukraine’s desperate attempts to repel its invader. The city has been under continuous bombardment by Russian artillery for more than two weeks.

There have been numerous reports of people running out of food, water and power. Other reports claim that some have resorted to eating pets to survive.

Russian news agencies said that buses have carried several hundred people Moscow calls “refugees” from Mariupol to Russia, although Ukraine maintains that they have been taken to Russia as hostages.

Mariupol City Council said that hundreds had died in the bombing of a theatre in the city earlier this month. Officials posted on Telegram: “Unfortunately, we start this day with bad news. From eyewitnesses, information appeared that about 300 people died in the drama theatre of Mariupol as a result of a bombardment by a Russian aircraft.”

A Mariupol City Council spokesperson said: “Until the last, I want to believe that everyone managed to escape. But the words of those who were inside the building at the time of this terrorist act say otherwise.”

US President Joe Biden’s national security adviser, Jake Sullivan, said the theatre bombing was an “absolute shock, particularly given the fact that it was so clearly a civilian target”.

Mircea Geoana, Nato’s deputy secretary general, said: “This is a barbaric war, and according to international conventions, deliberate attacks on civilians are war crimes.”

He said Putin’s efforts to break Ukraine’s will to resist are having the opposite effect: “What he’s getting in response is an even more determined Ukrainian army, and an ever more united west in supporting Ukraine.”

While the Russians continue to pound the capital, Kyiv, from the air, they appear to have gone into a “defensive crouch” outside the city and are focused more on the Donbas, a senior US official said, speaking on condition of anonymity.

“They don’t show any signs of being willing to move on Kyiv from the ground,” the official said. He added that the Russians were also no longer in full control of Kherson, the first major city to fall to Russian forces.

In another sign that Mr Putin’s forces are struggling, western officials say Russian soldiers drove a tank over one of their senior officers – Colonel Yuri Medvedev – after their unit suffered a massive number of losses in combat.

In Moscow, President Putin accused the west of trying to “cancel” Russian culture along with public figures including the Harry Potter author, JK Rowling. Ms Rowling hit back at the Russian leader for “slaughtering civilians”.

The Independent has a proud history of campaigning for the rights of the most vulnerable, and we first ran our Refugees Welcome campaign during the war in Syria in 2015.

Now, as we renew our campaign and launch this petition in the wake of the unfolding Ukrainian crisis, we are calling on the government to go further and faster to ensure help is delivered.

To find out more about our Refugees Welcome campaign, click here. To sign the petition click here. If you would like to donate then please click here for our GoFundMe page.