MOGADISHU - An explosion that occurred aboard a passenger plane after it took off from Somalia’s main airport on Tuesday, forcing it to make an emergency landing, was caused by a bomb, officials said Saturday, adding they had made arrests.

“Additional investigations conducted by Somali and international experts have confirmed the explosion that occurred inside the Daallo Airlines (jet) was not a technical problem but was a bomb that was intended to destroy the plane and kill all passengers onboard,” Somali Transport and Aviation Minister Ali Ahmed Jama told a press conference in Mogadishu.

“The security forces have detained people suspected of having involved the bomb that exploded inside that plane.” The blast punched a one-metre (three-foot)-sized hole in the side of the Airbus A321 about 15 minutes after it had taken off from Mogadishu heading for Djibouti.

A passenger identified as Abdulahi Abdisalam was killed, probably after being propelled out of the aircraft in the explosion, investigators said. Two out of around 60 passengers on board were slightly injured. The pilot, a 64-year-old Serb named Vladimir Vodopivec, told a friend that he was convinced the blast had been caused by a bomb, according to the Serbian daily Blic.

The explosion did not damage the plane’s navigation system, and this is what enabled the pilot to make the emergency landing, the report said. In their initial appraisal, the Somali authorities attributed the blast to sudden air decompression. Daallo Airlines operates out of Djibouti, flying to destinations in the Horn of Africa and the Gulf.

Somali government troops backed by African Union peacekeepers on Saturday recaptured the key port of Merka from Shebab insurgents, just one day after the Islamists swept into the city, the army and residents said. "The Somali forces and AMISOM peacekeepers secured control of Merka again and now the situation has returned to normal," a Somali military official, Abdirisak Mohamed, told AFP by telephone from Mogadishu.

"There was brief exchange of gunfire, but the (Shebab) militants have fled." He said "several" Shebab fighters and one Somali soldier were killed in the clashes, and "the security forces are now conducting clearing operations inside town." A resident reached by telephone said the AU force, known by its acronym AMISOM, used tanks to recapture the port, and four civilians were killed after they were caught in the fighting.

"Four people died in our neighbourhood and two others were wounded, I can see AMISOM and Somali troops returned to the city now and they are conducting security operations", Muhidin Osman said.

Another resident, Shamso Moalim, said "Shebab fighters pulled out of the town after putting up slight resistance, the Somali forces and AMISOM are back". "There are civilian casualties but I don't know the number", she added.

AU troops had fled the city, the state capital of Lower Shabelle, as heavily armed Shehab fighters swept in with black Islamist flags on Friday, residents and local authorities said, in one of the most dramatic reverses for the multi-national force in its nearly decade-long battle against the Shebab.