Kenyan troops are engaged in heavy fighting with the al-Shabab militant group on the coast of southern Somalia, both sides have said.
Al-Shabab spokesmen told news agencies that fierce clashes were under way in the port city of Kismayo.
Kenyan military spokesman Col Cyrus Oguna confirmed to the BBC that parts of Kismayo had been captured and the rest was expected to fall soon.
The Kenyan soldiers are part of the AU's Amisom force, which is trying to wrest control of Somalia for the newly elected, UN-backed president Hassan Sheikh Mohamud.
African Union forces have taken control of parts of the last major Islamist militant bastion in southern Somalia, Kenya's military says.
AU forces launched a beach assault in the port city of Kismayo, which has been a stronghold of the al-Qaeda-aligned group al-Shabab.