Ukrainian authorities say they found a mass grave containing 450 bodies in a northeastern city recaptured from Russian forces, calling it proof of war crimes carried out by the invaders in territory they had occupied for months.
"Russia is leaving death behind it everywhere and must be held responsible," Ukrainian President Volodymyr Zelenskyy said in a video address.
The mass grave discovered in the former Russian front-line stronghold of Izyum would be the biggest in Europe since the aftermath of the Balkan wars of the 1990s.
Ukrainian forces retook Izyum after thousands of Russian troops fled the area, abandoning weapons and ammunition.
The UN High Commissioner for Human Rights' office said it wanted to send a team to Izyum to verify the allegations of a mass grave.
"Our colleagues in Ukraine are following up on these allegations, and they are aiming at organising a monitoring visit to Izyum to determine the circumstances of the death of these individuals," spokeswoman Elizabeth Throssell said.
She added that the team was hoping to visit the northeastern city "soon".
"For months a rampant terror, violence, torture and mass murders were in the occupied territories," Mr Zelenskyy's adviser, Mykhailo Podolyak, tweeted in English above a photo of a forest scattered with wooden crosses in fresh muddy ground.
"Anyone else wants to 'freeze the war' instead of sending tanks? We have no right to leave people alone with the Evil."
Russia did not immediately comment on the reports of the mass grave. In the past it has denied its troops commit atrocities. Moscow calls its actions in Ukraine a "special military operation" to disarm its neighbour.
In Kupiansk, a northeastern railway junction city whose partial capture by Kyiv's forces on Saturday cut Russia's supply lines and led to the swift collapse of its front lines in the region, small units of Ukrainian troops were securing a nearly deserted ghost city.
Blood on the floor
A formerly Russian-occupied police station had been hastily abandoned. Russian flags and a portrait of President Vladimir Putin lay on the floor amid broken glass. Records had been torched. Behind the steel doors of the station's jail cells there was blood on the floor and stains on the mattresses.
Three piglets escaped from an abandoned sty were foraging in the city street. Serhiy, a middle-aged man in a thin jacket, was hungry for news.
"There’s no electricity, no phones. If there were electricity, at least we could have watched TV. If there were phones, we could have called our relatives," he said.
"If only there hadn't been all this bombing with everyone in their basements."
After a week of rapid gains in the northeast, Ukrainian officials have sought to dampen expectations that they could continue to advance at that pace.
They say Russian troops that fled the Kharkiv region are now digging in and planning to defend territory in neighbouring Luhansk and Donetsk provinces.
"It is of course extremely encouraging to see that Ukrainian armed forces have been able to take back territory and also strike behind Russian lines," NATO Secretary-General Jens Stoltenberg told BBC radio.
"At the same time, we need to understand that this is not the beginning of the end of the war. We need to be prepared for the long haul."
Mr Putin has yet to comment publicly on the battlefield setback suffered by his forces this month. Ukrainian officials say 9,000 sq km have been retaken, territory about the size of the island of Cyprus.
The speed of the advance has bolstered Ukrainian morale and bolstered its case for more weapons from Western allies.
In Washington, US President Joe Biden announced a new A$897 million (US$600 million) arms package for Ukraine, including High Mobility Artillery Rocket Systems (HIMARS) and artillery rounds.
The United States has sent about A$22.6 billion (US$15.1 billion) in security assistance to Kyiv since Russia's 24 February invasion.
Multiple areas hit by shelling
Russian forces shelled Kharkiv, Ukraine's second largest city, and other towns in the northeast on Thursday, Ukrainian officials said.
More than 90 missiles and artillery shells hit the neighbouring Sumy region on the border with Russia, according to its governor, Dmytro Zhyvytsky.
On the Russian side, Vyacheslav Gladkov, governor of Russia's Belgorod region, said the Ukrainian army shelled the town of Valuyki near the border.
"Anti-aircraft defences went into action but there is some destruction on the ground," he said on Telegram. This included a power substation put out of commission and private homes and vehicles set on fire.
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Reuters was not able to verify the reports.
In Uzbekistan on Thursday, Mr Putin met Chinese President Xi Jinping for the first time since the two men signed a "no limits" friendship pact three weeks before the start of the war.
In his public remarks, Mr Putin gave a rare hint of friction with Beijing over the war.
"We understand your questions and concern about this. During today's meeting, we will of course explain our position," he told Mr Xi.
Mr Xi did not mention Ukraine in his public remarks, nor was it mentioned in a Chinese account of the meeting. China has so far trod a careful path, condemning Western sanctions against Russia but stopping short of endorsing or assisting Moscow's military effort.
