MOGADISHU, Somalia — Somalia’s al-Shabab rebels withdrew from the southern Somali city of Kismayo overnight, the rebel group and residents said Saturday, a day after Kenyan and Somali government forces attacked the militants’ last bastion.

“We moved out our fighters … from Kismayo at midnight,” al-Shabab spokesman, Sheikh Ali Mohamud Rage, told Reuters.

He threatened to strike back soon. “The enemies have not yet entered the town. Let them enter  Kismayo which will soon turn into a battlefield,” he said.

African Union troops from Kenya, Uganda and Burundi have combined over the last 18 months to kick al-Shabab out of the Somali capital Mogadishu and take a series of smaller towns that the insurgents fled to.

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The assault is likely to send al-Shabab fighters underground. Hardcore fighters may unleash suicide bombs and ambushes but less dedicated fighters could melt back into their communities, further reducing al-Shabab’s strength.