Ukraine's military chiefs are facing growing criticism over a recent Russian missile strike that is believed to have caused heavy Ukrainian casualties.
Ukrainian media and Russian military bloggers say more than 20 Ukrainian soldiers were killed at Friday's award ceremony near the southern frontlines.
Ukraine's military has not yet given casualty figures of what it says was a "tragedy" in the Zaporizhzhia region.
President Volodymyr Zelensky said the incident "could have been avoided".
"Criminal proceedings have been initiated," the Ukrainian leader added in a post on social media on Sunday.
"Every soldier in the combat zone - in the enemy's line of fire and aerial reconnaissance - knows how to behave in the open, how to ensure safety."
A number of Ukrainian soldiers and military experts say the ceremony should not have taken place in a strike-risk area.
They say Ukrainian officers should have been aware that Russian drones are constantly monitoring Ukrainian troops' activities near the frontlines to guide air and artillery strikes.
Drone footage has now emerged on a Russian Telegram channel purportedly showing the moment of the deadly strike on what looked like a open-air ceremony.
A number of bodies believed to be those of Ukrainian soldiers are also seen lying on the ground.
Russia's military has not officially commented on the attack.
Ukrainian Defence Minister Rustem Umerov on Saturday confirmed earlier media reports that Ukrainian soldiers from 128th Mountain Assault Brigade "Zakarpattia (Transcarpathia)" were killed on Friday.
In a statement, he also ordered a "full investigation" in what he described as a "tragedy".
Meanwhile, Ukraine's Strategic Command (StratCom) said an Iskander-M - Russia's short-range ballistic missile - was used in the attack. A number of civilians were injured.
On Sunday, Ukrainian lawmaker Mykhailo Volynets said 28 people were killed and 53 injured in the attack, but this has not been officially confirmed.
President Zelensky said he wanted to "establish the complete truth about what happened and prevent such incidents from happening again".
The 128th Brigade is yet to officially comment on the issue.
Three days of mourning have been declared in Ukraine's westernmost Transcarpathia region, where many of the victims are believed to be from.
Kyiv has not publicly revealed the location of the strike, but reports in Ukrainian media say it was a village near the frontline.
Russian bloggers said it was the village of Dymytrovo (renamed to Zarichne by Ukraine in 2016).
The attack is believed to have happened as Ukrainian troops were marking Artillery Day, celebrating military personnel working in artillery and missile units.
Ruslan Kahanets, commander of Ukraine's volunteer battalion Sonechko (Sun), said in a Facebook post on Saturday that there was "a pile of dead officers and soldiers" after the Russian strike.
He also posted photos of burned vehicles and the bodies of killed soldiers.
A video has also emerged of a Ukrainian soldier, believed to be from a brigade located nearby, who publicly criticises officers for organising the reported award ceremony.
The unnamed soldier says that frontline villages are being hit "methodically and regularly" and "anyone who is here will tell you this".
"As a result of this [ceremony] line-up, many Ukrainian defenders and civilians died".
He asks what the officers who had gathered the crowd were thinking, because "everyone on the frontlines knows that a crowd of more than two people always provokes an 'arrival' ['strike' in military jargon]".
Serhiy Sternenko, a well-known Ukrainian volunteer, suggested on Saturday that the commander who had organised the ceremony should be jailed for life.
"There have already been many similar incidents. Unfortunately. Without systemic changes, there will be more such incidents," he said.
A number of social media users in Ukraine have been voicing their anger, demanding punishment for the organisers of the award ceremony.
"Who gathered them there, why is this person's name being withheld? Whose initiative was it? Are these people already under investigation?" one user said.
Another one wrote: "How was it possible to gather ALL our warriors in one place?"
Meanwhile, Russian military bloggers say more than 20 Ukrainian soldiers were killed.
One blogger on Sunday suggested that Ukraine's military chiefs now should "think why such incidents have become more frequent".
Another pro-Kremlin blogger wrote that "earlier, Ukrainians had 'punished' Russians in a similar way several times. And we quickly forgot about lining up outside, stopped huddling and began to constantly look at the sky".
