Ukraine has offered to release Russian prisoners of war in exchange for the safe evacuation of the badly injured fighters trapped inside a steel mill in the ruined city of Mariupol.

While fighting raged in Ukraine’s east and south, Ukrainian Deputy Prime Minister Iryna Vereshchuk said on Wednesday that negotiations were underway to release the injured fighters who are holed up in the last bastion of Ukrainian resistance in Mariupol. She said there were different options, but “none of them is ideal”.

Those negotiations are happening as Kyiv prepares for its first war crimes trial of a captured Russian soldier.

Ukraine’s top prosecutor said her office charged Vadin Shyshimarin, 21, in the killing of an unarmed 62-year-old civilian who was gunned down while riding a bicycle in February, four days into the war.

Shyshimarin, who served with a tank unit, was accused of firing through a car window on the man in the northeastern village of Chupakhivka.

Prosecutor general Iryna Venediktova said the soldier could get up to 15 years in prison. She did not say when his trial would start. Ms Venediktova’s office has said it has been investigating more than 10,700 allegations of war crimes committed by Russian forces and has identified over 600 suspects.

Many of the atrocities came to light last month after Moscow’s forces aborted their bid to capture Kyiv and withdrew from around the capital, exposing mass graves and streets and yards strewn with bodies in towns such as Bucha. Residents told of killings, burnings, rape, torture and dismemberment.

On the economic front, Ukraine shut down a pipeline that carries Russian gas across Ukraine to homes and industries in Western Europe, marking the first time since the start of the war that Kyiv disrupted the westward flow of one of Moscow’s most lucrative exports.

The move was made, Ukraine’s natural gas pipeline operator said, to stop Russian gas flowing through a station in part of eastern Ukraine controlled by Moscow-backed separatists because enemy forces were interfering with the station’s operation and siphoning gas.

The immediate effect is likely to be limited, in part because Russia can divert the gas to another pipeline and because Europe relies on a variety of suppliers.

In the southern Kherson region, site of the first major Ukrainian city to fall in the war, a Moscow-appointed leader said officials there want Russian President Vladimir Putin to annex the area.

Kirill Stremousov, deputy head of the Kherson regional administration appointed by Moscow, told Russia’s RIA Novosti news agency: “The city of Kherson is Russia.”

That raised the possibility that the Kremlin would seek to break off another piece of Ukraine as it tries to salvage an invasion gone awry.

Russia annexed Ukraine’s Crimean Peninsula, which borders the Kherson region, after a disputed referendum in 2014, a move denounced as illegal and rejected by most of the international community.

A Black Sea port of roughly 300,000, Kherson provides Crimea with access to fresh water and is seen as a gateway to wider Russian control over southern Ukraine.

Kremlin spokesman Dmitry Peskov said it would be up to residents in the region to decide whether an appeal to annex should be made. He said any move to annex territory would have to be closely evaluated by legal experts to make sure it is “absolutely legitimate, as it was with Crimea”.

Ukrainian presidential adviser Mykhailo Podolyak tweeted: “The invaders may ask to join even Mars or Jupiter. The Ukrainian army will liberate Kherson, no matter what games with words they play.”

On the battlefield, Ukrainian officials said a Russian rocket attack targeted an area around Zaporizhzhia, destroying unspecified infrastructure.

There were no immediate reports of casualties. The southeastern city has been a refuge for civilians fleeing the devastated port city of Mariupol.

Russian forces continued to pound the steel plant in Mariupol, its defenders said. The Azov Regiment said on social media that Russian forces carried out 38 airstrikes in the previous 24 hours on the grounds of the Azovstal steelworks.

The plant has sheltered hundreds of Ukrainian troops and civilians during a months-long siege.

An adviser to the Mariupol mayor said Russian forces have blocked all evacuation routes out of the city. Petro Andriushchenko said there are few apartment buildings fit to live in and little food or drinking water. He said some remaining residents are cooperating with occupying Russian forces in exchange for food.