By Feisal Omar and Abdi Sheikh

MOGADISHU Aug 26 (Reuters) - Islamist al Shabaab militants killed at least seven people including a regional official and local police commander in an attack on a government convoy in southern Somalia on Wednesday, officials and the group said.

Al Shabaab attacked the convoy between Garbaharey and Baladhawo towns in the Gedo region, near the Kenyan border.

The al Qaeda-allied group frequently launches attacks on officials in its bid to topple the Western-backed government and impose its strict interpretation of Islam on the nation, which is struggling to rebuild after two decades of war.

The group now controls increasingly smaller patches of territory since an African Union force and Somali troops drove it out of major strongholds in an offensive launched last year.

"We ambushed them but it turned into a fierce battle later,"

al Shabaab's military operations spokesman, Sheikh Abdiasis Abu Musab, told Reuters. He said 18 people were killed.

Senior Somali police officer Elmi Nur said three militants and seven others, including a deputy district commissioner and a regional police commander, were killed.

"We have been launching operations to eliminate al Shabaab from the region. More police and military were sent after the ambush to chase the fighters hiding in the forested areas," he said.

Al Shabaab often cites higher numbers for those killed than official figures. (Editing by Edmund Blair and Andrew Heavens)