Ukraine has vowed to find the Russian soldiers who apparently killed an unarmed Ukrainian prisoner of war, after graphic footage emerged online.

"We will find the murderers," President Volodymyr Zelensky said late on Monday.

In the video, the Ukrainian soldier is seen smoking a cigarette in a trench. He says "glory to Ukraine" before being shot with automatic weapons.

The soldier has not been officially named, but the BBC has spoken to a family who say they recognised him.

A woman, who said she was the soldier's sister, said: "My brother would certainly be capable of standing up to the Russians like that.

"He never hid the truth in his life and certainly wouldn't do so in front of the enemy."

The soldier served in the 30th Brigade of Ukraine's armed forces, the BBC has confirmed.

In the footage, one of the shooters - believed to be a Russian soldier - is heard saying "die" and using an expletive after the prisoner of war (POW) is shot dead.

The alleged killer or killers - who are not seen in the clip - have not been identified.

The BBC has not verified where and when the footage was made. It first emerged on social media on Monday. Russia has not commented.

Kyiv and its Western allies have accused Russian troops of committing mass war crimes since President Vladimir Putin launched a full scale-invasion of Ukraine on 24 February 2022. Russia denies the allegations.

In his video address, President Zelensky said "the occupiers" killed "a warrior who bravely said to their faces: 'Glory to Ukraine!'"

"I want us all to respond to his words together, in unity: 'Glory to the Hero! Glory to the Heroes! Glory to Ukraine!'"

Mr Zelensky was referring to a battle cry in Ukraine's military that has become popular among millions of Ukrainians.

Meanwhile, Ukrainian Foreign Minister Dmytro Kuleba tweeted that the footage was "another proof this was is genocidal", calling for an "immediate investigation" by the International Criminal Court.

Ukraine's human rights ombudsman Dmytro Lubinets accused Russian troops of committing war crimes by violating the Geneva Conventions regarding the treatment of POWs.

And Ukraine's Prosecutor General Andriy Kostin said a criminal investigation had already been opened.

Ukraine has previously accused Russian troops of torturing, raping and killing Ukrainian prisoners of war.

Last July, a video emerged that showed a captured Ukrainian soldier being castrated in the Russian-occupied Donbas region in eastern Ukraine.

The Russian soldier filmed carrying out the attack in the town of Severodonetsk was identified as a member of a unit belonging to Chechen leader Ramzan Kadyrov.

In November, Moscow accused Ukrainian forces of executing a group of Russian prisoners.

It followed a video from the front line in eastern Ukraine showing the apparent surrender of a number of soldiers, in an incident that ended in their deaths.

A Ukrainian official said the surrender was "staged" by Russian troops in an attempt to attack their captors.