A loud explosion was heard as Somali government troops together with African Union (AU) forces established control of the port city of Kismayo.
Residents said the blast sounded like a roadside bomb but caused no casualties.
The apparent attack came after the Islamist al-Shabab militia, linked to al-Qaeda, pulled out of Kismayo, its last major stronghold, at the weekend.
The army said all strategic points had been captured including the airport, port and main government buildings.
An al-Shabab spokesman, Sheikh Abdiasis Abu Musab, told Reuters news agency that the explosion had been caused by a bomb planted in a district administration office building.
“This is only an introduction to the forthcoming explosions,” he said.
There had been concerns that the fleeing militants had set a trap for government forces who entered the outskirts of Kismayo on Monday.
A Kenyan military spokesman said that improvised explosive devices had been planted by al-Shabab at Kismayo’s airport but that they had been detonated by bomb experts.
Once the new airport was safe then the first military aircraft could be expected to land there, Maj Emmanuel Chirchir tweeted.
The commander of the Somali government army in the Juba region, Ismael Sahardid, told BBC News on Tuesday that his troops and those of the AU force, known as Amisom, had secured all government installations.
“We have full control of the city. The residents of the city have welcomed us warmly,” he said.
People in Kismayo said that the Somali national flag had been hoisted by the army over the governor’s office, and that government soldiers were patrolling the streets on foot.
Checkpoints had been set up, an eyewitness told the BBC, and whenever military vehicles went past, residents would cheer and wave the Somali flag at them.
Al-Shabab were forced out of the capital, Mogadishu, in August 2011 by African Union troops who, with government forces, went on to take control of most of the militant-held towns.
The militants had used Kismayo as their main base for more than a year but moved out on Saturday as Somali forces backed up by Kenyan AU troops advanced towards the port city.
Al-Shabab fighters are still highly active in southern and central Somalia and have carried out suicide bombings and other attacks in towns they no longer control.
Source BBC