(Updates with new attacks)

By Ahmed Mohamed

BAIDOA, Somalia, April 3 (Reuters) - Somali Islamists killed at least 18 people in attacks across the Horn of Africa nation on Thursday, in a growing campaign of hit-and-run assaults spreading their rebellion far from its Mogadishu epicentre.

The al Shabaab militant group, which the United States says has al Qaeda links and put on its terrorist list in February, hit in three locations on Thursday, and took over an empty town.

Security experts say al Shabaab is leading the insurgency against President Abdullahi Yusuf's interim government, which defeated the militant group and its allies in the Somalia Islamic Courts Council with Ethiopian military help last year.

The bulk of the fighting takes place in the anarchic coastal capital Mogadishu, where the insurgents are waging a campaign of Iraq-style assassinations, roadside bombings and artillery attacks against the government and its Ethiopian allies.

In the deadliest of Thursday's strikes, al Shabaab attacked a local government-allied militia near the town of Adado, about 550 km (340 miles) north of Mogadishu. Government troops had long departed the area.

Residents said at least 15 people including three civilians were killed when the insurgents attacked with machine guns and rocket-propelled grenades. Many residents fled.

"Al Shabaab attacked Adado district, killing 12 local forces that were allied with the government. They also seized eight battle wagons and burned three tanks," resident Mohammed Moalim told Reuters by phone. "Dead bodies are littering the streets."

'BATTLE WAGONS'

Battle wagons, also known as technicals, are pick-up trucks mounted with heavy weapons that serve as troop transports.

In another attack, Al Shabaab struck Qansah Dheere, 60 km (40 miles) southwest of the interim government's base in Baidoa.

"Early this morning, a heavily armed al Shabaab group ambushed us, killing three of my soldiers, wounding two others and blowing up one of our cars," Bay region Governor Abdifitah Mohamed Ibrahim told Reuters by phone.

Baidoa is in south-central Somalia and remains one of the government's strongest bases, with an Ethiopian military presence to protect it and its airfield.

In nearby Bakol region, suspected Islamists gunned down a senior intelligence officer, witnesses said.

Al Shabaab spokesman Mukhtar Abu Mansur declined to comment on those attacks, but confirmed that his fighters had taken over Jalalaqsi, a town 180 km (110 miles) north of Mogadishu that has no government troops.

At least 6,500 people have died in Mogadishu since the government took over the city in December 2006, thousands more were wounded and more than 600,000 have fled cycles of attacks and reprisals that have pulverised a city already in ruins.

Since early December, al Shabaab fighters or local gunmen claiming allegiance to the group have attacked far-flung outposts and held the towns briefly.

With Somalia's shifting allegiances, it is often difficult to tell whether it is part of a concerted campaign or clan or money disputes al Shabaab has taken credit for, diplomats say.

Al Shabaab was the feared military wing of the court coalition that controlled most of southern Somalia in the second half of 2006, imposing strict Islamic law and threatening Yusuf's plans to impose his government's authority.

The United States has long feared the vacuum of authority in Somalia -- without an effective central government since 1991 -- will allow al Qeada to plan and carry out attacks in the rest of eastern Africa. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ ) (Additional reporting by Aweys Yusuf in Mogadishu, Writing by Bryson Hull; Editing by Giles Elgood)

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