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.
