The Moldova-flagged tanker Millennial Spirit has now been struck twice since Russia invaded Ukraine on Feb. 24.
It said a Russian Kh-31 air-to-surface missile had struck the ship.
"Probably the remainder of the cargo is burning," the military said in a statement cited by Interfax-Ukraine saying the tanker had been hit for a second time.
It was first hit with a missile fired from a warship days after the invasion. Moldova said at the time that the ship had a Russian crew and two were seriously wounded.
"Probably the remainder of the cargo is burning," the military said in a statement cited by Interfax-Ukraine saying the tanker had been hit for a second time.
It called the vessel a "delayed-action ecological bomb" and blamed the fact it was drifting without a crew on a Russian blockade of Ukrainian ports.
Details of the attack could not immediately be confirmed independently by Reuters.
Ukraine's southern military command said when the vessel was struck in February it had more than 500 tonnes of diesel on board and that since then it had been drifting without a crew.
Kyiv called the vessel a "delayed-action ecological bomb" and blamed the fact it was drifting without a crew on a Russian blockade of Ukrainian ports. It said a Russian Kh-31 air-to-surface missile had struck the ship.