Buulo Burte, 206km north of the capital, Mogadishu, was the second town opposition fighters captured within a week, and the sixth in recent months.
Last Wednesday, fighters took control of the Jowhar township, 90km north of the capital. After freeing prisoners and routing government forces, the fighters typically withdraw voluntarily.
In Mogadishu, meanwhile, doctors said at least four people have been killed in fighting between remnant fighters of the Islamic Courts' Union (ICU) and Somali transitional government soldiers alongside Ethiopian forces.
The transitional government took over Mogadishu in December 2006 with the help of Ethiopian tanks and air support, ousting the Islamic courts that had ruled southern Somalia for six months.
Opposition leaders, including the ICU, later formed an alliance after meeting in neighbouring Eritrea, and vowed to overthrow the transitional government.
Mogadishu and its outlying towns have, as a result, seen almost daily violence over the past year.
The guerrilla war has left hundreds of civilians dead and forced tens of thousands more to flee.
The UN High Commissioner for Refugees (UNHCR) recently said relentless violence in Somalia had forced at least 15,000 people to flee to neighbouring states since the start of the year.
Some 8,000 Somalis have fled to Kenya, 4,000 to Ethiopia, 2,000 to Djibouti and 1,300 made their way into Sudan, the UN said in a statement.
Somalia has had no effective government since Mohamed Siad Barre was ousted from power in 1991, paving the way for factional and tribal divisions that continue to destabilise the country.