The attack in the district of Ahwar targeted the command centre of the 111th Brigade, the security source said, adding that the commander of the brigade was "seriously" wounded in the blast.
The sources gave an initial toll of six dead, but said later that it had doubled to 12 dead. The commander was among the six wounded.
There was no immediate claim of responsibility for the attack.
Last month a gun battle in Abyan province left two suspected Al-Qaeda militants and a pro-government militiaman dead, after a failed attack on the head of a militia that backs the government.
