The Al-Qaeda-linked rebels, blamed by Nairobi for several kidnappings of foreigners on Kenyan soil, pose a significant security threat to the Horn of Africa region and Nairobi launched an air-and-ground offensive against them a week ago.

The rebels have reinforced their defenses in the town of Afmadow, a strategic transit point for goods trafficked illegally through Kismayu, Al-Shabaab's center of operations.

Somalia's Western-backed troops say the aim of the operation is to rid Kismayu of the militants, which if achieved, would wipe out their base for logistics and recruitment.

"A jet bombarded an Al-Shabaab base near the port. It dropped a huge shell, flew past, came and then dropped another shell," Kismayu resident Mahmoud Hassan told Reuters.

"The whole town shook. We've never heard anything like it. Everyone ran away," he said. A laborer at the harbor said the militants had ordered people to run to their homes.

"Al-Shabaab fighters deserted the port but besieged the areas around the base," Hared Ali told Reuters.

An Al-Shabaab official said a Kenyan jet had struck two bases in Kismayu.

"There were no casualties. We fired at the plane after the second bombardment and it has not come back," the senior official told Reuters from southern Somalia.

The Kenyan military said a French naval gunship had bombarded the town of Kuday, south of Kismayu on Saturday.

Gen. Yusuf Hussen Dhumal, commander of Somali government troops near Afmadow, told Reuters the town of Qoqani had been captured and they were heading to Afmadow and then Kismayu.

"Our troops in Taabto and Hayo have also moved near Afmadow and are just 7 km away. We wish in the coming two days to reach Afmadow ... Kenyan convoys are also with us," he told Reuters.

Residents said convoys of armored vehicles and trucks carrying weaponry, food supplies and tents were seen leaving four military camps in Isiolo in northern Kenya on Friday and heading towards the border.

Kenya says it has not encountered any resistance from the rebels and that the militants are on the run and getting weaker, but any attempt to take Afmadow, where the rebels have massed and dug trenches, may result in a significant ground battle.

Kenya is the latest of Somalia's neighbors to intervene militarily in a country that has not had an effective government for the last 20 years. The militants have vowed to bring "flames of war" into Kenya if Nairobi refuses to withdraw its troops.