A heavy blast rocked the Somali port city of Kismayo on Tuesday as Kenyan troops from the African Union force and their allies entered the former Islamist bastion, residents and the Shebab militia said.
"It was a huge explosion, and it looked like a roadside bomb," said resident Ahmed Jama, adding that it was not clear if there were casualties.
The explosion was claimed by the Al-Qaeda linked Shebab, which said in a message that the attack had targeted "apostate militia" and boasted of leaving "scores dead and injured".
The blast occurred as AU troops and allied Somali fighters made a wary advance into the strategic southern town, a key base for Shebab islamists until they abandoned their positions there Saturday.
"Allied forces have fully entered, and taken control of Kismayo today, all key areas including the airport, the harbour and main police stations were secured," said Somali commander Ismail Sahardid.
"We have arrested several suspected Al-Shebab members and security operations are going on to ensure calm... we have called on all clan fighters to lay down their weapons."
The port had been the de facto capital of the Shehab insurgents for over a year, but the fighters said they had pulled out on Saturday after Kenyan troops with the AU force launched a beach assault.
Kenyan forces have also pushed in by land from 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.
In Kismayo, a seaside town of some 160,000-190,000 people, the majority of residents were reported to be hiding inside their homes as the troops made careful progress, amid fears of booby traps or guerrilla attacks.