Experts say the Aris 13 sent a distress call, turned off its tracking system and altered course for the Somali coast before disappearing off radars.
Pirates have hijacked a Sri Lankan-flagged oil tanker, a Somali official said on Tuesday.
"The pirates hijacked the oil tanker and they brought it near Alula," Mohamud Ahmed Eynab, the district commissioner for Alula, said. Pirates in the port town confirmed they were expecting the ship.
The hijack is the first time since 2012 Somali pirates have successfully taken a commercial ship.
John Steed of the aid group Oceans Beyond Piracy, said the Sri Lankan-flagged Aris 13 oil tanker sent a distress call on Monday, turned off its tracking system and altered course for Alula.
Steed, a former British colonel and piracy expert, said the ship is thought to have eight crew.
"The ship reported it was being followed by two skiffs yesterday afternoon. Then it disappeared," he said.
TRT World's Oliver Whitfield-Miocic has more details.
Search operation underway
Aircraft from regional naval force EU Navfor were flying overhead to track the ship's progress and try to determine what was happening, Steed said.
The 1,800 tonne 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.
The ship was being monitored by the United Kingdom Maritime Trade Organisation (UKMTO), which coordinates the management of all merchant ships and yachts in the Gulf of Aden area, said Noel Choong, head of the International Maritime Bureau's piracy reporting centre in Kuala Lumpur.
The UKMTO in Dubai said it had no further information "at the moment."
Pirate attacks fall sharply after 2011
In their heyday five years ago, Somali pirates terrorised sailors crossing the Gulf of Aden.
They launched 237 attacks off the coast of Somalia in 2011, the International Maritime Bureau says, and held hundreds of hostages.
That year, Ocean's Beyond Piracy estimated the global cost of piracy was around $7 billion. The shipping industry bore around 80 percent of those costs, the aid group's analysis showed.
But attacks fell sharply after ship owners tightened security and avoided the Somali coast.
Intervention by regional naval forces that flooded into the area helped disrupt several hijack bids and secure the strategic trade route that leads through the Suez Canal and links the oil fields of the Middle East with European ports.