Kenyan forces have attacked the Somali port city of Kismayo, in an effort to drive out al-Qaida-linked militant group al-Shabab.

Kenyan military spokesman Colonel Cyrus Oguna said Friday's early morning attack included both maritime and air forces in coordination with African Union and Somali government troops.

Oguna said Kenyan forces took control of the city “with minimum resistance.” However, al-Shabab said on Twitter that “Kismayo remains firmly in the hands” of its fighters, and witnesses report ongoing fighting.

The African Union force in Somalia, AMISOM, urged all fighters in Kismayo to lay down their arms and promised safety to those who give up peacefully.

Kismayo is the last major stronghold of al-Shabab, which has used the town's seaport to bring in weapons and other supplies.

Kenyan forces entered Somalia last year after a series of cross-border kidnappings that Kenya blamed on the militant group.

Al-Shabab is seeking to overthrow Somalia's government and impose a hardline form of Islamic law in the country.

The group once controlled most of southern and central Somalia, but has lost most of its territory over the past 18 months to pro-government forces.