Kenyan forces launched an amphibious assault on the Somali port city of Kismayo on Friday, in a long-delayed offensive aimed at depriving al Qaeda-linked militants of their last stronghold in East Africa.

The African Union-backed drive appears to have surprised al Shabaab militants as well as residents of Kismayo who were hoping to flee before serious fighting began. Kenyan troops seized the city's port, according to the African Union Mission in Somalia, or Amisom. Other areas of control remained unclear. As of late Friday,...