Al-Qaeda-backed Islamist insurgents claimed to have killed 63 Kenyan soldiers in an attack on an African Union military base in southwestern Somalia and captured the town of El-Adde near the border with Kenya.

Sheikh Abdulaziz Abu Musab, a spokesman for the military wing of al-Shabaab, made the claim on Radio Andalus, a pro-militant station that broadcasts via the Internet in the capital, Mogadishu. The attack is the biggest assault on an African Union base since a raid on an installation at Janaale in southern Somalia on Sept. 1.

The African Union Mission in Somalia “can confirm that there was an attack on our troops in El-Adde” and more details will be released later, it said Friday on its Twitter account. Kenya Defence Forces spokesman David Obonyo said the militants attacked a Somali National Army camp close to a Kenya Defence Forces camp at 4 a.m. Friday. The KDF has “increased its military operations” against al-Shabaab in a bid to contain the situation, and the number of casualties on both sides is still being calculated, he said.

Al-Shabaab has waged an insurgency in Somalia since 2006 to impose its version of Islamic law. It’s used the country as a staging ground to carry out attacks in Kenya, East Africa’s largest economy, and Uganda. While the militant group has lost territory since being driven out of Mogadishu in 2011 by government and African Union forces, it continues to stage deadly gun and bomb attacks, including in the capital.

The African Union Mission in Somalia has deployed about 22,000 peacekeepers to help Somali government forces stabilise the country, which hasn’t had a functioning central administration since civil war erupted a quarter of a century ago. Kenyan forces form part of the Amisom contingent, along with troops from Ethiopia, Burundi, Uganda and Djibouti.

In the attack on Janaale, the group claimed to have killed 80 African Union soldiers. Amisom said at the time it was verifying the number of casualties. The mission is estimated to have lost at least 1,100 troops since 2009, the Nairobi-based East African newspaper reported in September.

In Friday’s raid, Abu Musab said the militants detonated a vehicle laden with explosives at the fence of the base in El-Adde and fired rocket-propelled grenades at the troops inside. Equipment and ammunition were seized in the raid.

The battle lasted for about three hours as Kenya Defence Forces troops tried to repel the attack, before the base was overrun, Mohamed Isaq, a resident of El-Adde, said by phone from the town.

•Text courtesy of Bloomberg. Photo showsAfrican Union peacekeepers but not the ones attacked by the Islamists.