Ukrainian President Volodymyr Zelensky has demanded answers for the families of soldiers killed in a missile strike during an awards ceremony on Friday.

A Ukrainian unit said 19 of its soldiers were killed in a Russian attack near the front lines in the southern Zaporizhzhia region.

The 128th Mountain Assault Brigade said its "best fighters have been killed".

Many have expressed disbelief that the ceremony was allowed to go ahead so near to the front line.

President 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 appears to be an 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 had earlier confirmed reports that soldiers from the 128th Mountain Assault Brigade had been killed in the strike.

He ordered a "full investigation" into what he described as a "tragedy".

Meanwhile, Ukraine's Strategic Command (StratCom) said an Iskander-M - a short-range ballistic missile used by Russia - was used in the attack. A number of civilians were injured.

In his own statement, President Zelensky said he wanted to "establish the complete truth about what happened and prevent such incidents from happening again".

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 front line.

Russian bloggers said it was the village of Dymytrovo - which was renamed Zarichne by Ukraine in 2016.

The attack is believed to have been launched as Ukrainian troops marked Artillery Day, which celebrates 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" in the aftermath. He also posted photos of burned vehicles and soldiers' bodies.

A video has also emerged in which a Ukrainian soldier - believed to be from a nearby brigade - publicly criticises officers for organising the reported ceremony.

The unnamed soldier says that front-line 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 front lines knows that a crowd of more than two people always provokes an 'arrival' [air strike]".

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 also voiced their anger and demanded punishment for the ceremony's organisers.

"Who gathered them there, why is this person's name being withheld? Whose initiative was it? Are these people already under investigation?" one user wrote.

Another asked: "How was it possible to gather ALL our warriors in one place?"

Meanwhile, a Russian military blogger suggested that Ukraine's military chiefs should now "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".