A Ukrainian source told CNN that a sea drone with 450 kilograms of TNT had attacked the ship and claimed there were about 100 Russian servicemen aboard.
“A big navy ship Olenogorsky Gornyak was hit,” the source told CNN. “As the result of the attack, the Russian ship has received serious damage and is not able to fulfill its duties.”
The vessel was identified by the Ukrainian source and Russian military bloggers as the Olenogorsky Gornyak, an amphibious Russian landing ship.
EDIT: Wikipedia uses the Romanization "Olenegorsky Gornyak". I have no idea what the difference is, but just to improve searchability, thought I'd mention it.