Somali forces clashed with al-Shabab fighters overnight Tuesday in the East African country’s Lower Shebelle region. A deadly firefight broke out when the Islamic militant group ambushed Somali National Army troops stationed at a base in the embattled southern port city of Merca, according to Somali radio and television station Shabelle Media Network.

Government sources have not yet disclosed the exact number of casualties from Tuesday night’s battle in Merca, the provincial capital of Lower Shebelle which is located some 60 miles from the national capital, Mogadishu. Al-Shabab recaptured Merca last month, making it the largest city under the group’s control, according to BBC News.

The loss of the coastal city was a major setback for the African Union Mission to Somalia, or Amisom, the continental bloc’s peacekeeping mission in the country, in its decade-long fight against the Somalia-based terrorist organization. Regaining control of Merca, which faces the Indian Ocean, grants al-Shabab access to a port again and will provide a financial boost to the group's operations.

Amisom and the Somali National Army had held the key port city for 3 1/2 years after retaking it from al-Shabab in August 2012, when the troops expelled the militants from a number of Somali towns. More than 20,000 African Union troops are currently deployed in the Horn of Africa nation. But their efforts against al-Shabab have been hindered by a lack of air power that has left their bases and supply lines vulnerable.

“It’s symbolically important because it was one of the major towns retaken from al-Shabab and underlines the resilience and longevity of the group,” Ahmed Soliman, a regional analyst at Chatham House in London, told the Guardian. “Al-Shabab has the ability to absorb losing control of key towns and have consistently been able to regroup in rural areas as well as strike in urban areas.”

Al-Shabab, a Somalia-based terrorist group, emerged in 2006 from the now-defunct Islamic Courts Union that controlled Mogadishu. The al Qaeda affiliate launched its own insurgency on major Somali cities in 2009, controlling Mogadishu and southern Somalia until it was pushed out by domestic and international forces around 2012. Many areas of south-central Somalia are still under al-Shabab’s control, and the Sunni extremist group has increased efforts in recent months to recapture lost territories while seeking to topple Somalia’s Western-backed government.