Somalia's al-Shabab terror group on Sunday launched an attack on a base for African Union troops in the country's south, killing four Ugandan peacekeepers, said Ugandan army spokesman Brig. 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.

Local sources said a massive blast was heard in the Bulomarer district, some 150 kilometers (93 miles) south of Mogadishu, and fighting broke out after dozens of heavily armed al-Shabab militants stormed the base.

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.

"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-Shabab 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-Shabab's casualty figures are often different from those issued by Somali officials.

The Al-Qaida linked al-Shabab was pushed out of the Somali capital in 2011 — and subsequently other towns and cities — by soldiers from the African Union Mission in Somalia (AMISOM).

But the group still holds sway in large parts of the countryside and launch regular gun and bomb attacks on government, military and civilian targets in Mogadishu and ambushes on military convoys and outposts.

East African leaders contributing to the 22,000-strong AMISOM force last month called for the United Nations to reconsider plans to withdraw troops by December 2020, saying the timeline was unrealistic and could lead to a reversal of gains.