Four other guards were wounded in the attack on the convoy of the head of the security services in the town of Loder in Abyan province, the source said.
Al-Qaeda and the rival Islamic State group have exploited a power vacuum created by the conflict between the government and Huthi rebels to expand their presence in Yemen, especially in the south.
Al-Qaeda fighters were driven out of towns and cities in Abyan province in September by government forces backed by a Saudi-led coalition, fleeing to hard-to-reach areas.