SANAA, Yemen -- Security officials say gunmen have ambushed a military convoy in south Yemen, killing nine soldiers.

The killings in the southern Abyan province brought to 13 the number of troops killed Sunday. Four soldiers were killed earlier when gunmen attacked the presidential compound in Taiz, Yemen's second-largest city. The attack was blamed on a group established to avenge the killing of protesters at the hands of President Ali Abdullah Saleh's security forces.

It was not immediately clear who was behind the attack in Abyan, according to the officials, who spoke on condition of anonymity because they were not authorized to speak to the media.

The attacks came after Saleh left for Saudi Arabia for medical treatment after he was wounded in a rocket attack on his Sanaa compound on Friday.