African Union troops and allied Somali forces bolstered positions Wednesday in the former Islamist bastion of Kismayo, residents said, a day after entering the Somali port city amid heavy explosions.

Wary shopkeepers, who had shut down business since Al-Qaeda linked Shebab abandoned the city on Saturday, reopened for the first time, as patrols of Somali and AU forces moved through the town.

"There was shooting last night and a curfew in place until dawn, but today things have been quiet, no fighting, and we hope the situation is returning to normal," said shopkeeper Abdirasak Hassan.

"I have opened my shop but business is not so busy, as many people are staying in their homes to see if the (AU and Somali) forces are well disciplined."

The hardline Shehab claimed responsibility for a heavy blast on Tuesday, while AU forces detonated several homemade bombs discovered as they seized control of the town's airport and harbour.

Attackers also hurled a grenade into a tea shop late Tuesday, wounding seven people, according to residents.

Long lines of Kenyan troops – part of the AU mission – marched in force into the southern port city on Tuesday, alongside a mix of Somali militia forces crammed into pickup trucks mounted with machine guns.

"KDF (Kenyan Defence Forces) and SNA (Somali National Army) troops continue to consolidate the gains," the Kenyan army said Wednesday.

Somali forces also include the Ras Kamboni militia, commanded by former warlord Ahmed Madobe, who switched from supporting the Shebab to fighting alongside the AU.

"Somali soldiers, mostly fighters from Ras Kamboni, are patrolling the streets, and have set up checkpoints stopping vehicles and are making security checks," said Ahmadey Abdulahi, a resident.

Residents in Kismayo – relatively peaceful under four years of authoritarian rule by the Shebab – are reportedly fearful of violence between rival Somali clans in potential power struggles for control of the port.

While the Ras Kamboni are largely drawn from the Ogadeni clan, powerful forces from the Marehan as well as the Majertan clan are also influential in the region.

Kenyan forces have also pushed in by land from the west, where they have trekked in for some 200 kilometres (125 miles) after an invasion force of troops and tanks crossed the border last year, before later joining the 17,000-strong AU force.

The Shebab used Kismayo as an export hub for the illegal charcoal trade. Its loss is a major blow to the fighters, and marks the latest in a string of defeats since they abandoned fixed positions in the capital Mogadishu last year.

The hardline insurgents still control the town of Jowhar, some 500 kilometres (300 miles) northeast of Kismayo, and one small port town, Barawe, lying some 230 kilometres (140 miles) up the coast from Kismayo.