Russian-controlled regions of eastern and southern Ukraine have announced plans to start voting to become integral parts of Russia.

The concerted and quickening Kremlin-backed efforts to swallow up four regions could set the stage for Moscow to escalate the war against Ukrainian forces successfully battling to wrest back territory.

The announcements of referendums starting on Friday in the Donetsk, Luhansk, Kherson and partly Russian-controlled Zaporizhzhia regions came after a close ally of Russian President Vladimir Putin said votes were needed, as Moscow loses ground in the war that began nearly seven months ago.

Former president Dmitry Medvedev said folding regions into Russia itself would make redrawn frontiers “irreversible” and enable Moscow to use “any means” to defend them.

Ukrainian foreign minister Dmytro Kuleba denounced the votes as a sham and tweeted that “Ukraine has every right to liberate its territories and will keep liberating them whatever Russia has to say”.

US national security adviser Jake Sullivan said: “We will never recognise this territory as anything other than part of Ukraine,” adding that they reflect Russia’s setbacks on the battlefield.

“These are not the actions of a confident country. These are not acts of strength,” he said.

German Chancellor Olaf Scholz said: “It is very, very clear that these sham referendums cannot be accepted.”

The votes, in territory Russia already controls, are expected to go Moscow’s way but are unlikely to be recognised by western governments backing Ukraine with military and other support.

Luhansk and Donetsk form much of the Donbas region, which has been gripped by separatist fighting since 2014 and which Mr Putin has set as a primary objective of the Russian invasion.

In Donetsk, separatist leader Denis Pushilin said the “long-suffering people of the Donbas have earned the right to be part of the great country that they always considered their motherland”.

He added that the vote will help “restore historic justice that millions of the Russian people were waiting for”.

Pressure within Russia and from Moscow-backed leaders in Russian-controlled regions of Ukraine for votes to pave their way to becoming Russian increased after a Ukrainian counteroffensive — bolstered by western-supplied weaponry — that is recapturing large areas of Russian-occupied territory.

Kremlin spokesman Dmitry Peskov said on Tuesday that there are no prospects for a diplomatic settlement.

Mr Medvedev, the deputy head of Russia’s Security Council chaired by Mr Putin, said that votes in separatist regions are important to protect their residents and “restore historic justice” and would “completely change” Russia’s future trajectory.

“After they are held and the new territories are taken into Russia’s fold, a geopolitical transformation of the world will become irreversible,” said Mr Medvedev, who served as Russia’s president from 2008 to 2012.

“An encroachment on the territory of Russia is a crime that would warrant any means of self-defence,” he said, adding that Russia would enshrine the new territories in its constitution so no future Russian leader could hand them back.

“That is why they fear those referendums so much in Kyiv and in the West,” Mr Medvedev said. “That is why they must be held.”

The recapturing of large areas of previously Russian-occupied territory, most notably in the north-eastern Kharkiv region, has strengthened Ukraine’s arguments that its troops could deliver more stinging defeats to Russia with additional armament deliveries.

More heavy weaponry is on its way, with Slovenia this week promising 28 tanks and Germany pledging four additional self-propelled howitzers.

More aid also is expected from the UK, already one of Ukraine’s biggest military backers after the US. Prime Minister Liz Truss is expected to promise that in 2023, her government will “match or exceed” the £2.3 billion in military aid given to Ukraine this year.

The swiftness of the Ukrainian counteroffensive also saw Russian forces abandon armoured vehicles and other weapons as they beat hasty retreats.

Ukrainian forces are recycling captured weaponry back into battle. Washington-based think tank the Institute for the Study of War said abandoned Russian T-72 tanks are being used by Ukrainian forces seeking to push onward into Russian-occupied Luhansk.

In the counteroffensive’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 that officials found many bodies “with signs of violent death”.

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

Ukrainian officials have also alleged Russian forces tortured 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 the capital Kyiv at the start of the war.