The United Kingdom Maritime Trade Operations centre issued an alert over the attack, warning ships in the area.
The private security firm Ambrey also said that an attack was under way, saying it targeted a Malta-flagged tanker heading from Sikka, India, to Durban, South Africa.
Ambrey said it appeared to be an assault by Somali pirates, who have been reported as operating in the area in recent days and who seized an Iranian fishing boat to use as a base of operations.
Iran has not acknowledged the fishing boat's seizure, called the Issamohamadi.
Details of the attacked vessel correspond to the Hellas Aphrodite, which changed its track and slowed down at the time of the attack.
The ship's owners and managers could not immediately be reached for comment.
Another maritime security firm, the Diaplous Group, said the attacked tanker had a crew of 24 mariners, all of whom reportedly locked themselves into the ship's citadel for safety during the attack.
The vessel did not have an armed security team aboard it, the firm said.
The European Union's Operation Atalanta, a counter-piracy mission around the Horn of Africa, did not immediately respond to a request for comment.
That EU force has responded to other recent pirate attacks in the area and had issued a recent alert to shippers that a pirate group was operating off Somalia and assaults were "almost certain" to happen.
Thursday's attack comes after another vessel, the Cayman Islands-flagged Stolt Sagaland, found itself targeted in a suspected pirate attack that included both its armed security force and the attackers shooting at each other, the EU force said.
Piracy off the Somali coast peaked in 2011, when 237 attacks were reported.
Somali piracy in the region in 2011 cost the world's economy some $US7 billion, with $US160 million paid out in ransoms, according to the Oceans Beyond Piracy monitoring group.
The threat was diminished by increased international naval patrols, a strengthening central government in Somalia, and other efforts.
However, Somali pirate attacks have resumed at a greater pace over the last year, in part due to the insecurity caused by Yemen's Houthi rebels launching attacks in the Red Sea corridor over the Israel-Hamas war in the Gaza Strip.
In 2024, there were seven reported incidents off Somalia, according to the International Maritime Bureau.
So far in 2025, multiple fishing boats have been seized by Somali pirates.
