• He was named on Tuesday by the Ukrainian military as Tymofiy Shadura, who had been missing since 3 February.
• The 30th Separate Mechanised Brigade said he was last seen near the eastern city of Bakhmut - the scene of fierce fighting in recent months.
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.
He was named on Tuesday by the Ukrainian military as Tymofiy Shadura, who had been missing since 3 February.
The 30th Separate Mechanised Brigade said he was last seen near the eastern city of Bakhmut - the scene of fierce fighting in recent months.
"Currently, the body of our serviceman is in the temporarily occupied territory," the statement said, citing preliminary information. It added the identification could be completed after the body was returned.
Before the soldier was named, Mr Shadura's sister told the BBC: "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."
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, or how the soldier was captured. It first emerged on social media on Monday.
Russia has not publicly commented on the incident.
The general staff of Ukraine's armed forces said: "The shooting of an unarmed prisoner is a cynical and brazen disregard for the norms of international humanitarian law and the customs of war. This is what worthless murderers do, not warriors.
"The Russian occupiers have once again shown that their main goal in Ukraine is the brutal extermination of Ukrainians."
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 war 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.