* Hijack is first such incident since 2012
* Eight crew on Sri Lankan ship Aris 13
* Pirates launched 237 attacks off Somalia in 2011-officials (Updates with Somali region sending troops)
By Katharine Houreld
NAIROBI, March 14 (Reuters) - Pirates have hijacked an oil tanker with eight Sri Lankan crew on board, Somali authorities said on Tuesday, the first time a commercial ship has been seized in the region since 2012.
Security forces have been sent to free the Aris 13, a regional police official said late on Tuesday.
"We are determined to rescue the ship and its crew. Our forces have set off to Alula. It is our duty to rescue ships hijacked by pirates and we shall rescue it," Abdirahman Mohamud Hassan, director general of Puntland’s marine police forces, told Reuters by phone.
Puntland is a semi-autonomous northern region of Somalia. Alula is a port town there where pirates have taken the tanker.
Experts said the ship was an easy target and ship owners were becoming lax after a long period of calm.
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 force declined to comment.
The pirates brought the ship to the port town of Alula, district commissioner Mohamud Ahmed Eynab told Reuters by phone.
A pirate called Abdullahi told Reuters by telephone: "We are now heading on boats towards our colleagues holding the ship at Alula. We are carrying water, food and weapons for reinforcement."
The Sri Lankan government said eight Sri Lankan crew were onboard and the ship 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 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.
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.
That year, Ocean's Beyond Piracy estimated the global cost of piracy was about $7 billion. The shipping industry bore roughly 80 percent of those costs, the group's analysis showed.
But attacks fell sharply after ship owners tightened security and vessels stayed further away from the Somali coast.
There had only been four attempted attacks by Somali pirates in the past three years, the bureau said.
Intervention by regional naval forces that flooded into the area helped disrupt several hijack bids and improved security for the strategic trade route that leads through the Suez Canal and links the oilfields of the Middle East with European ports.
Before Tuesday's hijacking, only one crew remained captive in Somalia. The crew of 17 Iranians was taken two years ago, but four are believed to have died, four were rescued and one escaped, Steed said, so only eight remained.
"They're from Baluchistan in Iran," he said, referring to a violent and restive eastern province. "No one really cares about them."
(Additional reporting by Keith Wallis in Singapore, Abdiqani Hassan in Bosaso, Ranga Sirilal in Columbo and Jonathan Saul in London; Editing by Alison Williams)