BILOGORIVKA, Ukraine (AFP) - The charred basement at the bottom of a heap of rubble that was once a two-storey grade school was still hot a week after a military strike Ukraine says killed 60 people sheltering from Russian bombs.

Three of the building's soot-covered corners were all that stood upright at the site of what Kyiv alleges was one of the gravest crimes committed by Russian forces since the Feb 24 invasion of their pro-Western neighbour.

Cracks of close-range mortar fire echoed across the smouldering remains of other buildings and military equipment littering the roads of Bilogorivka - an east Ukrainian village razed by one of the biggest battles of the war.

Mr Vladimir Gerasimenko climbed from his cellar during the briefest of lulls in fighting and walked out dazed to catch his first glimpse of the sun in seven days.

He could hardly believe his eyes.

"The world has turned upside down," the 70-year-old said while staring at the remains of a school the Soviets built for the coal mining village after World War II.

"Slavs are killing Slavs. Who knows why or what for."

Local governor Sergiy Gaidai says 60 people sheltering in the school's basement "likely" died in a Russian aerial bombing attack on May 7.

Moscow's UN ambassador has rejected the "absurd" allegation that Russia was deliberately targeting schools.

But missile strikes and bombings of schools such as the one in Bilogorivka have become all too common across the war zone.

The United Nations children's relief fund estimates that one in six schools receiving its support in the Donbas war zone have been either "damaged or destroyed".

Most east Ukrainian schools and other big buildings have been turned into emergency relief centres or shelters that are often filled with both soldiers and civilians.

Yet the ruins on School Street in Bilogorivka revealed few secrets about what exactly happened on that fateful night.

There were no signs of life or bodies in either the top layers of the rubble or the still-intact floors of the basement on the school's better-preserved side.

The impact left a massive crater in the middle of the building that appeared to run deep underground.

A golden thermal blanket hinted at a rescue effort that the governor says pulled out some survivors within the first few hours of the attack.

The strike occurred in the middle of a still-raging battle that began with Russian attempts to cross a strategic river running at the north end of the village three weeks ago.

Ukrainian forces have repelled waves of Russian attempts to land tanks on their side of the river using pontoon bridges.

The Russians have been able to make the crossing further west as part of a broader push on Ukraine's eastern administrative centre in Kramatorsk.

But Ukraine's ability to hold the line at Bilogorivka has thwarted Russia's bid to encircle neighbouring Severodonetsk and Lysychansk - two industrial cities forming the last pocket of resistance in the war zone's Lugansk region.

Both cities and the ruined village now stand completely deserted and deprived of water and other basic services.

"I would leave but there is no one to help me escape," Mr Gerasimenko's neighbour Margarita Kovalenko said.

"There are four of us left on this street that I know of," his wife Olga added.

"Us three - we spent the whole time together in the basement - and a young man down the street."

Bilogorivka had no visible Russian military presence despite rounds of mortar fire ringing out from the hills overlooking the village's northern edge near the school.

A car carrying a handful of Ukrainian soldiers pulled past a destroyed and abandoned checkpoint on a road leading into Bilogorivka and ground to a halt.

The silent troops positioned their long guns on top of their open car doors in a defensive posture and waited for signs of movement further down the road.

The remains of a destroyed Russian tank lay hidden among the debris on the opposite side of the street from Mr Gerasimenko's house.

The engineer by training looked at the 12 craters left behind by exploding shells in his back yard and said he might feel safer if the Russians actually seized the village for good.

"The Russians helped the Lugansk and Donetsk regions during the most difficult times," he said in reference to an eight-year insurgency that Moscow backed before outright invading Ukraine. "They sent in humanitarian assistance," he said.

"And all the others only sent in weapons. So I trust Russia more than all the other combined."