Mogadishu - African Union troops and tanks entered the former al Shabaab stronghold of Kismayu on Tuesday, but a blast claimed by the al Qaeda-linked militants who have fled the Somali port signalled their intention to fight back.
The government said the explosion caused no casualties, but the incident pointed to al Shabaab's capacity to hit back with guerrilla strikes and bombings in both Kismayu and neighbouring Kenya, whose troops led the assault on the town.
Residents and a government spokesman described a loud blast aimed at Somali soldiers patrolling Kismayu's dusty streets, but the rebels said they had detonated a bomb at a building housing Somali troops, "killing many".
"The bomb was planted inside a district administration office building," al Shabaab spokesman Sheikh Abdiasis Abu Musab said, warning of more attacks. "This is only an introduction to the forthcoming explosions."
The African Union forces entered Kismayu for the first time on Tuesday after launching an offensive against the port on Friday, forcing the rebels to flee. Hundreds of Somali government troops and allied militia fighters deployed in the city's winding streets on Monday.
Mohamud Farah, a spokesman for Somali government forces in the southern Juba region, said the militants had thrown a grenade at Somali troops, but there were no casualties.
"A hand grenade was hurled at one of our military cars passing in front of the district headquarter building. No casualties," he told Reuters from Kismayu.
"The grenade missed the car. We have seized one of the three al Shabaab suspects who were behind the grenade attack."
Although the successes against al Shabaab are welcomed by Somalia's government and its international backers, there are fears that even Kismayu's capture may not deliver a knock-out blow to the combat-hardened rebels.
The Islamist militant group, which merged with al Qaeda in February, has carried out a campaign of suicide bombings since it withdrew from the capital last year under military pressure.
Suspected al Shabaab militants threw a hand grenade at a Kenyan police post on Monday in the northern town of Garissa close to the border with Somalia, police said, a day after two police officers were shot dead in the town.
Kenyan authorities blame the rebels for a series of attacks in the east African country since it sent its troops into Somalia last October.
Residents in Kismayu spoke of two grenade blasts, then another big explosion.
"First two successive grenades were hurled at the Somali troops patrolling the middle city streets, the forces opened fire and then we heard a bigger explosion that shook the whole city in the direction of the district office building," a resident, Halima Farah, told Reuters.
Residents said the African Union troops fired in the air to disperse people who had gathered to watch the tanks rolling into the city for fear that militants would mount grenade attacks.
"AU tanks and Somali military vehicles have taken positions in Kismayu, people have welcomed us," General Ismail Sahardiid, the commander of Somali government forces, told Reuters.