Ukraine is using captured Russian tanks to solidify its gains in its north-east amid an ongoing counter-offensive, a US-based think tank said.

It comes as Kyiv vowed to push further into territories occupied by Moscow.

The Institute for the Study of War, citing a Russian claim, said Ukraine has been using left-behind Russian T-72 tanks as it tries to push into the Russian-occupied region of Luhansk.

“The initial panic of the counter-offensive led Russian troops to abandon higher-quality equipment in working order, rather than the more damaged equipment left behind by Russian forces retreating from Kyiv in April, further indicating the severity of the Russian rout,” the institute said.

Earlier this month, Ukraine launched its counter-offensive, pushing into territory around its second-largest city of Kharkiv.

Videos and photos showed Ukrainian troops seizing tanks, ammunition and other weaponry left behind by Moscow in an apparently chaotic withdrawal.

In the counter-offensive’s wake, Ukrainian officials found hundreds of graves near the once-occupied city of Izium.

Yevhenii Yenin, a deputy minister in Ukraine’s Internal Affairs Ministry, told a national telecast officials exhuming the dead found bodies “with signs of violent death”.

“There are many of them,” Mr Yenin said.

“These are broken ribs and broken heads, men with bound hands, broken jaws and severed genitalia.”

Ukrainian officials have also accused Russian forces of torturing people in occupied areas, including shocking them with radio telephones dating back to the Soviet era.

Russia has repeatedly denied abusing or killing prisoners, though Ukrainian officials found mass graves around the city of Bucha after blunting a Russian offensive targeting Kyiv at the start of the war.

Meanwhile, a Ukrainian push continues in the south of the country.

The Washington-based institute, citing the Ukrainian military, said Kyiv has destroyed ammunition depots, two command posts and an electronic warfare system.

Ukraine’s southern military command said early on Tuesday its troops sank a Russian barge carrying troops and weapons across the Dnipro River near the Russian-occupied city of Nova Kakhovka.

It offered no other details on the sinking of the barge in Ukraine’s Russian-occupied Kherson region, which has been a major target as part of Kyiv’s ongoing counter-offensive in the country.

In other developments:

— The website of a prominent Russian mercenary group was seemingly targeted by hackers. The Wagner Group’s website could not be reached on Tuesday morning. Late on Monday, the IT Army of Ukraine, a group of hackers supporting Kyiv, posted a screenshot apparently showing the website had been replaced with images of dead Russian soldiers.

— Moscow has likely moved its Kilo-class submarines from their station on the Crimean Peninsula to southern Russia over fears they could be struck by long-range Ukrainian fire, the British military said on Tuesday. In a daily intelligence briefing, the Ministry of Defence said the submarines had “almost certainly” been moved to Krasnodar Krai in mainland Russia, instead of a naval base at Sevastopol on the Crimean Peninsula.

“This is highly likely due to the recent change in the local security threat level in the face of increased Ukrainian long-range strike capability,” it said. “In the last two months, the fleet headquarters and its main naval aviation airfield have been attacked.”