Fighting and shelling was taking place on the beaches of al-Shabab stronghold Kismayo, in southern Somalia, after an assault by Kenyan and Somali troops.
The loss of the port would deal a huge blow to al-Shabab. It is a lucrative source of revenue and a centre for operations over areas the militia has controlled in south-central Somalia since 2007.
The group, which formally merged with al-Qaeda in February, has been steadily losing its footholds under sustained pressure from African peacekeeping forces (AMISOM) and Somali government troops for the past year.
While Kismayo's recapture would go a long way towards stabilising Somalia, which has lacked effective central government for the past 20 years, it is thought that it could also embolden the militants to resort to more guerrilla-style attacks.