A blast rocked a vegetable market in Somalia's third-largest city, killing at least twelve people and wounding at least 30, officials said Monday, and the country's top militant group claimed responsibility.

Prime Minister Abdiweli Mohamed Ali slammed the group for the bombing.

"What they did was contrary to Islam and our culture ... We are committed to clear (Al Shabab) out of the country."

Baidoa police officer Abdullahi Ahmed said the blast was caused by a roadside bomb, while witness Nur Yusuf said the bomb was concealed in a thermos placed along the street where he sells vegetables.

"So far, we know eight people died and fifteen others were injured," Ahmed said. "The death toll may rise, but it was a cowardly attack targeting civilians _ we have arrested one suspect after the blast."

Eyewitness Fadumo Haji said she saw the carnage.

"They killed innocent and poor people," she said.

The al-Shabab militant group claimed responsibility for the attack. Baidoa was a major base for the group until Ethiopian troops and Somali soldiers seized the key town in February.

"The explosion has targeted Ethiopians and their apostate companions," the group said in a statement posted on its website.

It continued: "Then they opened fire at the civilians in the market, killing five people on the spot."

Some 100 soldiers are being sent to Baidoa as an advance team for 2,500 troops soon to be deployed there. The African Union troops will be stationed alongside Ethiopian troops already in Baidoa.

A blast last week in the capital ripped through the national theater, killing 10 people and wounding many more. Among the dead were two top sports officials.