A car bomb has exploded in Baidoa, where Somalia's fragile interim government is based, leaving at least six people dead.
A government official has blamed al-Qaeda
A policeman told the BBC that a female suicide bomber wearing a veil blew herself up at a check-point.
The explosion also destroyed two other cars. "There were flames everywhere," an eye-witness said.
President Abdullahi Yusuf survived a suicide car bomb attack in Baidoa two months ago, which killed his brother.
He blamed that attack on his Islamist rivals, who denied responsibility.
There are fears of widespread conflict breaking out in Somalia between the government and the Union of Islamic Courts (UIC), which controls most of the south of the country, including the capital, Mogadishu.
There have been conflicting reports of how many bombs there were.
Deputy Defence Minister Salad Ali Jelle told the AP news agency that three car bombs had exploded at police check-points, killing the drivers and three others.
A local journalist said he had seen three bodies.
Police commander General Ali Hussein told AFP that at least 12 people had died.
"There were two suicide cars full of explosives," he said.
Mr Salad also told AP that three alleged attackers had been captured.
He said they were foreign members of al-Qaeda.
The UIC denies links to al-Qaeda but is opposed to the government and has threatened to launch a holy war to drive Ethiopian troops out of the country.
Ethiopia admits it has hundreds of military trainers helping the government but denies they are taking part in any conflict.
The Ethiopian parliament on Thursday passed a resolution authorising the government to take all necessary and legal steps against any invasion by UIC.
The resolution said there was a clear and present danger to Ethiopia from the UIC.