Somalia's al-Shabaab Islamic extremists on Sunday launched an attack on a base for African Union troops in the country's south, killing four Ugandan peacekeepers, said Ugandan army spokesperson Brigadier Richard Karemire.Four other Ugandans were injured, Karemire told The Associated Press. At least 22 of the attackers were killed and the rest were repelled, he said.The attack started in the morning when two suicide car bombs exploded at the entrance to the heavily fortified base in Bulo-Marer, an agricultural town in Somalia's Lower Shabelle region, said Somali army Col. Ahmed Hassan.Nearly 100 fighters firing propelled grenades and machine guns attacked the base occupied by the Ugandan soldiers."That attack was repulsed but several soldiers have lost their lives to the bomb," said Hassan.Residents reported explosions and sustained gunfire, sending hundreds of terrified villagers to the bush.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. "We had a frightening day but the situation is calm now," said Nur Yusuf, a local elder reached by phone, who added that African Union and Somali troops were patrolling the town in the evening.Al-Shabaab claimed responsibility for the attack. The group said that 14 of its fighters were killed during the attack on the African Union base, according to Sheikh Abdulaziz Abu Musab, the group's military spokesman. Al-Shabaab's casualty figures are often different from those issued by Somali officials.Al-Shabaab, which is fighting to impose Shariah law across Somalia, was pushed out of the capital, Mogadishu, and other major urban cities more than two years ago, but the group still carries out suicide attacks across Somalia.