(CNN) -- Troops from the African Union and Somali army say they made significant gains Friday in an assault on one of the final strongholds of an al Qaeda-linked militant group in the nation.
AMISOM, the African Union force, said that its soldiers had successfully entered the port city of Kismayo in the morning. It said more forces are on their way.
Kismayo, south of the capital of Mogadishu, is a key battleground between African Union forces and Al-Shabaab, an Islamic group that has long tried to overthrow the Somali government, but has been increasingly beleaguered.
"We urge all fighters remaining in Kismayo to lay down their arms," said Lt.-Gen. Andrew Gutti, the commander of the African Union force.
The Kenyan Defense Force, which is part of the offensive, said on its Twitter account that its troops were expanding into the rest of the city.
The Kenyan and Somali forces mounted "a fierce assault on air, sea and land" to take Kismayo, said Col. Adan Ahmed Rufle, the spokesman of the Somali army in Lower Juba, the region around the city.
He said the Al-Shabaab fighters were in "disarray" and some had been killed. He was unable to provide precise numbers of casualties.
But the militant group said it was still in control of the port city.
"Kismayo remains firmly in the hands of the Mujahideen, stay tuned for updates," said a posting on a Twitter account believed to be controlled by the Al-Shabaab.
The looming fight for Kismayo prompted thousands of civilians to flee the city this month, according to the U.N. refugee agency. It said that Somalis were leaving in minibuses, in trucks and on donkey carts.
Kenyan forces said last week they were using land, sea and air forces to attack the militant group before the Kismayo offensive.
Military analysts believe, however, that Al-Shabaab is militarily cornered. After losing control of much of Mogadishu last year, Kismayo became even more critical to the militant group. Al-Shabaab uses the port to garner much needed tax revenue from the illegal charcoal trade and smuggling operations.
Since crossing into Somalia in October, Kenyan commanders and politicians have stated more than once that capturing Kismayo was the ultimate aim of their operation.
CNN's Lillian Leposo and journalist Mohamed Amiin Adow contributed to this report.