At least four civilians were killed and nine others injured in Somalia on Thursday by mortar shells that were likely targeting peacekeeping force AMISOM's base at the airport in the capital Mogadishu, witnesses told Reuters.

The mortar shells fell short of their likely target, striking homes just outside a sprawling compound that houses both the AMISOM base and airport, according to witnesses.

"As we took a break for tea, mortar shells landed...one landed on my neighbour," killing two people and injuring five others, 65-year-old Mohamed Sheikh Mohamed, an area resident, told Reuters.

Mohamed and other witnesses told Reuters they believed the mortars had been aiming for the AMISOM base, long a target of similar attacks.

There was no immediate claim of responsibility. Somalia's Islamist group al Shabaab has, in the past, conducted similar attacks on the base and airport.

Among those killed were a man and his son, while the man's two other children, a 3-month-old girl and 4-year-old boy, and their mother were injured.

"Immediately another shell landed on my daughter's house just in the vicinity," Mohamed said, adding his daughter was injured in the blast while her husband and their son were killed.

Government and AMISOM officials did not respond to Reuters' calls or text messages seeking comment. A police spokesman did not immediately provide comment.

In a news report on its website, Somali's state news agency said an attack by mortar shells had killed a man and his son and injured five others of the same family, but did not say anything about other casualties that witnesses had reported.

The al Qaeda-allied al Shabaab has been fighting for more than ten years to try to topple Somalia's internationally backed central government and establish its own rule based on its strict interpretation of Islam's sharia law.

Somalia has been gripped by Islamist violence and lawlessness since 1991.