NAIROBI, Kenya (CNN) -- Troops from the African Union and the Somali national army say they have made significant gains in an assault on one of the final strongholds of an al Qaeda-linked militant group in Somalia.

AMISOM, the African Union force, said that its soldiers had successfully entered the port city of Kismayo on Friday morning and that more forces were 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 AMISOM force.

The Kenyan Defense Force, which makes up the majority of troops in the offensive, said on its Twitter account that its troops had taken control of Kismayo and were expanding out into the rest of the city.

The Kenyan and Somali national army 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.

The looming fight for Kismayo had 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.

The Kenyan Defense Force (KDF) said last week it had been using land, sea and air forces to attack Al-Shabaab and degrade its forces before the offensive on Kismayo.

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 in Nairobi, and journalist Mohamed Amiin Adow in Stockholm, Sweden, contributed to this report.