Ukrainian authorities are just beginning to sift through the wreckage of the devastated city of Lyman in eastern Ukraine, assessing the humanitarian toll as well as possible war crimes committed during Russia's months-long occupation.

Few buildings in the city in the Donetsk region — an area which Moscow claimed as Russian territory following a staged referendum — survived without damage. Most houses are without basic utilities.

Walls around the town bear graffitied reminders of the four-month occupation by Russian troops, with words like "Russia", "USSR" and "Russian World" scrawled on surfaces that are riddled by bullets.

Mark Tkachenko, a communications inspector for the Kramatorsk district police of the Donetsk region, said authorities were still searching for the bodies of civilians and trying to determine causes of death.

They will look at when people died and how they died," he said.

"If it was in the period when the city was occupied and they have injuries from Kalashnikov rifles, then of course it's a war crime."

He said it was still unclear how many people had died in the city after it was overrun by Russian forces in May, but described Lyman today as a "humanitarian crisis".

"Some people died in their houses, some people died in the streets, and the bodies are now being sent to experts for examination," he said.

"For now, we are looking for grave sites, and there are probably mass graves."

The road approaching Lyman, which Russians used as a strategic logistics and transport hub during its occupation, is scarred by the intense fighting which took place as Ukrainian troops pressed to retake it late last week.

The forests surrounding the city are decimated, and the burned-out and twisted wreckage of dozens of vehicles line the road which is pockmarked by craters from falling rockets.

Tetyana Ignatchenko, a spokeswoman for the Donetsk regional administration, said the city's civilian infrastructure had been "completely destroyed", and work was underway to clear it of Russian soldiers' bodies which were abandoned during the army's retreat.

"Police and criminologists are working, looking for Russian bodies and collecting them in the streets and forests," Ms Ignatchenko said.

As they left Lyman, Russian soldiers placed mines on the bodies of some of their fallen comrades so they would explode when Ukrainian authorities attempted to clear them, Mr Tkachenko said. He said some had exploded but they had not caused injuries.

As Ukrainian authorities entered the city, they found many civilian residents had been killed by shelling, while others — mostly older people — had died during the Russian occupation because of a lack of food and medicine, Mr Tkachenko said.

He said Russian looting of civilian homes had been widespread.