The Kenyan army killed 21 al Shabaab militants in a town in southern Somalia after two of its own soldiers were killed in an ambush by the insurgents, a military spokesman said.

David Obonyo, spokesman for the Kenya Defence Forces, said that in the Saturday incident in Afmadow, an improvised explosive device hit and damaged one of their vehicles.

"Following the incident, 21 al Shabaab militants were killed, 19 AK 47 rifles, three rocket propelled grenades and a pistol were recovered," Obonyo said in a statement late on Saturday.

"Regrettably, KDF suffered two fatalities and five injuries. The injured were evacuated and are receiving medical attention."

Somalia's government is battling to rebuild the Horn of Africa nation after more than two decades of conflict. Al Shabaab ruled large parts of Somalia until 2011, when it was driven out of Mogadishu by African Union and Somali troops.

The militants, who aim to topple the Western-backed government, often inflate casualty numbers and downplay the number of their own fighters killed.

Abdiasis Abu Musab, al Shabaab's military operation spokesman, told Reuters on Saturday their fighters had killed 12 soldiers.

In January, Kenyan troops as part of the African Union Mission in Somalia (AMISOM) took heavy losses when al Shabaab launched a dawn raid on their camp in El Adde near the Kenyan border, although they have not disclosed exact casualty figures.

Al Shabaab claimed the attack had killed more than 100 soldiers.