(NAIROBI) A Somali official says eight Sri Lankan crewmembers were onboard when the tanker when the pirates attacked.
Experts said the ship was an easy target and ship owners were becoming lax after a long period without many attacks.
The Aris 13 sent a distress call on Monday, turned off its tracking system and altered course for the Somali port town of Alula, said John Steed of the aid group Oceans Beyond Piracy.
"The ship reported it was being followed by two skiffs yesterday afternoon. Then it disappeared," said Steed, an expert on piracy who is in contact with naval forces tracking the ship.
Aircraft from regional naval force EU Navfor were flying overhead to track the ship, he said.
"The pirates hijacked the oil tanker and they brought it near Alula," Mohamud Ahmed Eynab, the district commissioner for Alula, told Reuters on Tuesday by phone. Pirates in the town confirmed they were expecting the ship.
The Sri Lankan government said it had eight Sri Lankan crew onboard and flew a flag from the Comoros islands.
Data from Reuters systems showed it made a sharp turn just after it passed the Horn of Africa on its voyage from Djibouti to Mogadishu.
The 1,800 deadweight ton Aris 13 is owned by Panama company Armi Shipping and managed by Aurora Ship Management in the United Arab Emirates, according to the Equasis shipping data website, managed by the French transport ministry.
Graeme Gibbon-Brooks, the head of private maritime security company Dryad Maritime Intelligence, said the vessel was an easy target because it was low, slow and close to the coast.
Crews were beginning to relax their vigilance after a period of relative security for shipping, he said.
Now that the ship was captured, Somali authorities must ensure it was contained and not used as a mothership, he said, referring to a hijacked vessel used to launch attacks.
"The way that the authorities react now is crucial," he said.
In their heyday in 2011, Somali pirates launched 237 attacks off the coast of Somalia, data from the International Maritime Bureau showed, and held hundreds of hostages.