A U.N. humanitarian official for Somalia says fighting for control of al-Shabab's largest remaining stronghold appears “imminent.”

In a statement Tuesday, U.N. official Mark Bowden says he is “deeply concerned” about reports of civilian casualties in Port Kismayo caused by naval gunfire and airstrikes. He calls for all parties to minimize the conflict's impact on civilians.

Several African nations have troops in Somalia fighting al-Shabab, which is allied with al-Qaida.

Last Saturday, an unidentified naval ship fired on Kismayo, killing three people and injuring four.

The head of the U.N. Office for Humanitarian Affairs in Somalia says civilians could get trapped in the conflict.

“We have anticipated this one could be one of the larger battles and again, there's a large civilian population, there's a large IDP population and they are very likely to get caught up in that fighting,” said Justin Brady.

Kismayo is al-Shabab's most strategically important stronghold. The port gives al-Shabab access to the sea. And just south of the area, al-Shabab runs training camps in a large jungle area.

In June, Kenyan Prime Minister Raila Odinga said the Kenyan military hoped to capture Kismayo by August.

African Union and Somali government troops have made major gains this year against al-Shabab, which once controlled much of southern and central Somalia. Kenyan and Ethiopian forces are also in Somalia fighting the group.