The area has been devastated by catastrophic flooding along the Dnipro river after the Nova Kakhovka dam was blown up.

Kyiv and Moscow have accused each other of destroying the dam.

Oleksander Prokudin, the Ukrainian-appointed governor of the Kherson region, said on Ukrainian television that Russian forces were "deliberately trying to disrupt rescue efforts".

"Today, terrorists opened fire on three boats that were used to rescue 21 people from the flooded eastern bank," Mr Prokudin said.

"Almost all of them were elderly and people with limited mobility. Russian soldiers fired at these people in the back."

Two law enforcement officers were also among the victims.

The group was being evacuated from the Russian-controlled parts of Kherson region to parts controlled by Ukraine.

Ukrainian President Volodymyr Zelenskyy condemned the incident and repeated accusations that Russia was responsible for the breach of the dam.

Russian forces, he said, were now shelling residents seeking evacuation from submerged areas.

"[They have] cast people deliberately into flooded towns and villages and then shell boats used to try to evacuate them," President Zelenskyy said in his nightly video address.

"Even animals have more morals than the Russian state."

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Pictures posted on a Ukrainian armed forces Telegram channel showed paramedics taking evacuees out of small boats and rushing through knee-deep water to take them to safety - some on stretchers, some in rescuers' arms.

The shelling comes as Ukraine's forces said it had recaptured three villages from Russian troops in the southeast of the country.

The villages are the first liberated areas since Ukraine launched its long-awaited counteroffensive earlier this week.

An unverified video shows soldiers hoisting the Ukrainian flag at a bombed-out building in the village of Blahodatne in the Donetsk region.

The video was published by Ukraine's 68th Jaeger Brigade.

Deputy defence minister Hanna Maliar later said in a statement Kyiv's forces had also retaken Makarivka, the next village along, advancing between 300m and 1,500m (984ft to 4,921ft) in two directions on the southern front.

A Ukrainian territorial defence unit also posted unverified footage on Telegram of its soldiers holding up their flag in Neskuchne, the village that had been closest to Ukrainian positions in the area.

Separately, Russia and Ukraine announced the return of nearly 100 soldiers from each side.

Russia's Defence Ministry, in a Telegram post, said 94 Russians in Ukrainian captivity had been released after negotiations and would be taken to a medical institution to be examined.

Andriy Yermak, the head of Ukraine's presidential administration, said 95 Ukrainian service members had been returned, including some wounded. They included members of the national guard and border guards.