Hundreds found in mass burial site after Russians leave Ukraine city

* Zelenskiy blames Russia, says it must be held responsible
* Moscow has denied its troops carried out atrocities
* Blood in police cells abandoned by Russians
* Kyiv's advance encouraging but war not near end -NATO head

Ukrainian officials said they found a mass burial site with 440 bodies, mainly of civilians, 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 Zelenskiy said in a video address overnight.
The site in the former Russian front-line stronghold of Izium would be the biggest mass burial found in Europe since the aftermath of the 1990s Balkan wars. Ukrainian forces retook Izium after thousands of Russian troops fled the area, abandoning weapons and ammunition.
Ukrainian police chief Ihor Klymenko told a news conference all of the bodies recovered so far at the site appeared to be of civilians, although there was information that some soldiers might have been buried there too.
"For months a rampant terror, violence, torture and mass murders were in the occupied territories," Zelenskiy adviser Mykhailo Podolyak tweeted in English, above photos of a forest scattered with wooden crosses in fresh muddy ground. A huge pit was taped off with red-and-white crime-scene tape.
"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 burial site. In the past it has denied its troops commit atrocities in the conflict. 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 at the front, small units of Ukrainian troops were securing a nearly deserted ghost town.
A formerly Russian-occupied police station had been hastily abandoned in Kupiansk, about 60 km north of Izium.
Russian flags and a portrait of President Vladimir Putin lay on the floor of the station 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 on the loose 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."
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, about the size of the island of Cyprus.
Ukraine has also launched a major offensive to recapture territory in the south, where it aims to trap thousands of Russian troops cut off from supplies on the west bank of the Dnipro river, and retake Kherson, the only large city Russia has captured intact since the start of the war.
Russia's state-run RIA news agency released video showing smoke billowing from Kherson's Russian-occupied administration building after apparent Ukrainian rocket attacks.
Kirill Stremousov, the Russian-installed deputy head of the region, told Russian state TV that one wing of the building had been practically destroyed, and there were dead and wounded though it was too soon to say how many. Ukrainian officials did not immediately comment.
In the east, the chief prosecutor of the pro-Russian separatist administration in Luhansk was killed by an explosion in his office, along with his deputy, according to Russian news agencies. Russia also reported strikes across the border in its Belgorod region.
The war and sanctions on Russia have caused a surge in energy prices especially in Europe, which relies on Russian oil and gas. Germany announced on Friday a regulator was seizing the German arm of Russian oil company Rosneft, including a giant refinery supplying most fuel for the capital Berlin.
The Schwedt refinery depends on oil pumped from Russia through the "Friendship" pipeline to formerly Communist eastern Europe. German officials have said they expect the country will no longer receive Russian oil.
The speed of Ukraine's advance has boosted its morale and bolstered its case for more weapons from Western allies.
In Washington, US President Joe Biden announced a new $600 million arms package for Ukraine, including High Mobility Artillery Rocket Systems (HIMARS) and artillery rounds. The United States has sent about $15.1 billion in security assistance to Kyiv since Russia's Feb. 24 invasion.