"A suicide bomber wearing a heavy vest of explosives targeted the Muslim congregation at the Qaadim mosque in the center of the town as people were leaving the mosque after performing Friday prayers," Abdinasir Gulled told the dpa news agency.
Another 20 people were injured in the attack, which may have been aimed at a provincial politician, Gulled said.
Al-Shabab claimed responsibility for the attack on their Radio Andalus, Al Jazeera reported.
The armed group, which has ties to al-Qaeda, regularly launches attacks on government officials, journalists and prominent businessmen, as well as civilians, in the volatile nation in the Horn of Africa.
The incident comes after three people were killed and seven were injured in a suicide attack at a Mogadishu restaurant on Wednesday. Information ministry spokesman Ismail Mukhtar Omar said the attacker walked into the Blue Sky restaurant near Mogadishu National Theatre before blowing himself up.
At least three Somali special forces soldiers were also killed and an American officer was wounded in a car bombing and mortar attack outside a military base in Somalia's south on Monday. Al-Shabab said it was behind the attack and put the number of dead at 20.
A Somali soldier was also wounded in the attack in Janay Abdalla village, some 60km (37 miles) from Kismayo - which is also Jubaland state's administrative capital.
Somalia has faced near-continuous conflict for almost 30 years, while the internationally-backed government in Mogadishu has been fighting al-Shabab since 2008.
In August, 10 civilians and a police officer were killed in a gun-and-bomb attack by al-Shabab on an upscale beachfront hotel in the capital.
That same month, four al-Shabab fighters held in Mogadishu's central prison were killed in an intense shoot-out with security forces after they managed to get their hands on weapons within the facility.
At least seven people were killed in a separate attack in August when a car bomb exploded at an army base in Mogadishu where members of the Somali National Army (SNA) were stationed.