At least 51 people have been killed and 271 others injured in a Russian missile strike on the city of Poltava, in central Ukraine.
A military academy and a nearby hospital were hit. Ukraine's land forces confirmed that military personnel were killed in the attack.
People did not have enough time to get to bomb shelters after the air raid alarm sounded, Ukraine's ministry of defence said.
President Volodymyr Zelensky promised that what he called "Russian scum" would pay for the attack, and repeated calls for more air defences so that Ukraine could protect itself by carrying out its own long-range missile attacks. Moscow has not commented on the attack.
People nearby told us their windows were blown out by the force of the impact.
He described the moment the attack unfolded just after 09:00 local time on Tuesday morning (06:00 GMT), saying the second missile hit just three seconds after the first.
“I ran outside, there was smoke and dust everywhere…lots of people were outside having a cigarette, and many were killed…”
He says that what he saw has affected him psychologically, that there was “too much blood, too many dead bodies.”
Earlier reports from Russian military bloggers suggested that cadets were gathered at a military parade being held at the institute, but the ministry confirmed there was no parade happening at the time of the strike.
Alarms sounded at 09:08 local time and everyone headed to the shelter, the defence ministry added. The explosions started a few minutes after the alarms.
Thirty-year-old year old Jana Kulishova was one of them. She was woken by the air raid siren but did not have time to reach a shelter.
For her, the attack is personal. Her husband is fighting on the frontline in the Donbas region.
“Soldiers have died here and I know there are still soldiers still under the rubble. Their wives are waiting for them."
Ukrainian MP Oleksiy Goncharenko told the BBC the two minutes between the air raid siren in Poltava and the missiles landing was "nothing".
"You just imagine you're on the sixth floor of some building and you need to run away downstairs. Is it realistic that you can do this in two minutes?" he said.
"Just imagine this life and like this several times per day. We can't continue like this. It's just not fair."
In a statement on Telegram which confirmed the deaths of the military personnel, Ukraine's land forces said an investigation was under way to establish whether enough was done to protect those in the facility the missile hit.
It confirmed additional measures to ensure safety at military facilities would be increased.
Firefighters have been on the scene with all their equipment clearing out buildings, and army personnel outside have stopped people. They cannot get too close due to the sensitivity of the site.
Earlier, one Poltava resident, Olena Serdyuk, said: "The air raid alert started in just a minute, and then there were two explosions.
"At home, we immediately ran into the bathroom with the child, but then gathered ourselves".
She added that there is no bomb shelter "in our radius", and it "takes a long time to run somewhere".
Another local, Anastasiia Artyukh, heard "two very strong explosions" and called the situation "really scary".
"There is a house nearby, everything was blown out, all the windows. We don't have a basement in our house. So our only option was to sit [and wait]."
Poltava regional governor Philip Pronin called the attack a "cunning and cynical Russian strike," and later said that 15 people were still thought to be trapped under the rubble.
Mr Pronin said search groups were on site, and that 10 residential buildings had been damaged in the attack.
More details could not immediately be given due to security reasons, he said.
Ukraine's first lady Olena Zelenska posted on X calling the attack "a stunning tragedy for all of Ukraine," and added: "Russia is taking away the most valuable thing from us - life."
In a video on the Ukrainian presidential website, President Zelensky said Russia would be held accountable for the strike.
In the clip, which was also posted on X, Mr Zelensky said a "full and prompt" investigation had been ordered and that "all necessary services are involved in the rescue operation".
He went on to say that Ukraine needed long-range air defence systems "that can protect us from Russian terror" and added that "every day of delay, unfortunately, means more lost lives".
The UK's Foreign Secretary David Lammy condemned the "sickening act of aggression in Putin’s abhorrent and illegal war in Ukraine," and said "my thoughts are with all the victims and their loved ones."
US National Security Council spokesman John Kirby, meanwhile, described the attack as "another horrific reminder of Putin's brutality towards the people in Ukraine," and said that Washington would be sending Kyiv more military aid in the coming weeks.
Germany's Foreign Minister Annalena Baerbock said that Vladimir Putin "knows no limits to brutality".
There has also been criticism of the Ukrainian authorities in the aftermath of the attack.
Maria Bezugla, a Ukrainian MP who often criticizes the country's military leadership, accused officials of putting soldiers in danger.
"These tragedies keep repeating themselves. When will it stop?" she wrote on Telegram.
Since the attack, changes within the Ukrainian government have been announced - with potentially more to come.
The minister in charge of weapons reduction resigned on Tuesday and one senior lawmaker for Mr Zelensky's party said a "major government reset" was coming that would see more than half of minsters change.
"Tomorrow a day of sackings awaits us, and a day of appointments the day after," David Arakhamia said.
The attack on the military institute in Poltava came as Russian leader Vladimir Putin arrived in Mongolia - his first visit to an International Criminal Court (ICC) member since it issued a warrant for his arrest last year.
Poltava, located in central Ukraine, had a pre-war population of 300,000 and is located 300 kilometres (189 miles) east of Kyiv.
Founded in the 1960s, when Ukraine was still part of the USSR, the Poltava military communications institute trains telecommunications specialists.
Source: BBC