"No fewer than 200 soldiers were sleeping in the barracks" when Russian troops struck early Friday, a Ukrainian serviceman on the ground, 22-year-old Maxim, told AFP without providing his last name.

"At least 50 bodies have been recovered, but we do not know how many others are in the rubble," he said.

Another soldier estimated that the bombing could have killed around 100 people.

Authorities have not yet released an official death toll.

The military facility located in the north of the city was fully destroyed after being hit by several rockets Friday morning.

"Yesterday orcs hit our sleeping soldiers with a rocket in a cowardly manner," Vitaly Kim, head of the regional administration, said in a video on Saturday, using the Ukrainian nickname for Russian forces.

"A rescue operation is under way," he added. He did not provide further details, saying he expected to receive official information from the armed forces.

AFP journalists at the site earlier in the day saw the bodies of three people, including a shredded corpse extracted from under the rubble by rescue workers.

A survivor was also pulled out.

"We are continuing to count but it is impossible to know given the state of the bodies," a rescuer told AFP.

Mykolaiv's mayor Oleksandr Senkevych told local journalists that the city, which had a pre-war population of nearly half a million residents, had been struck from the neighbouring region of Kherson, now under Russian control.

For days, the Russians have bombarded Mykolaiv, which lies on the road to the strategic port city of Odessa some 130 kilometres (80 miles) down the Black Sea coastline.