( 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 . Thousands of Somalis flee as troops close in . 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 . Read more about Al-Shabaab on CNN 's Security Clearance blog . 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 . Related : Mogadishu bombing kills broadcaster . 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 . Related : Soldiers killed in blast targeting new president . CNN 's Lillian Leposo and journalist Mohamed Amiin Adow contributed to this report .
African Union forces successfully enter the port city of Kismayo . The city is a key battleground with Al-Shabaab , an Islamic group . Hide Caption 1 of 24 Photos:Attack at Kenyan university Students listen to an address by Joseph Nkaissery, Kenya's interior minister, on April 2, 2015.