Thirteen soldiers and two civilians were killed on Friday when the army fought gunmen in the city of Loder in south Yemen, a local security official told AFP.
Eight of the soldiers were killed when a rocket-propelled grenade hit their armored vehicle, Loder regional security chief Yahya al-Barkani said.
A security source said the armed men involved in the firefight may belong to al-Qaeda, while another local official said the gunmen may have links with a southern separatist movement.
Friday's fighting followed a two-hour clash on Thursday in Loder in which gunmen killed two soldiers and wounded two others.
The fighting broke out after the assailants attacked two soldiers in the marketplace and seized their weapons, leading to the intervention of the army.
South Yemen is feared to have become a base for al-Qaeda militants to regroup, under the network's local franchise, al-Qaeda in the Arabian Peninsula (AQAP).
The south is also the site of frequent protests and separatist unrest with southerners complaining of discrimination by the Sanaa government over the allocation of resources.
It was independent from 1967 until 1990 when it united with the north. It seceded in 1994, sparking a short-lived conflict which ended with the south bring overrun by northern troops.
Western powers fear that al-Qaeda is exploiting instability in Yemen, which borders top oil exporter Saudi Arabia and sits next to a strategic shipping lane, to strengthen its operations and launch attacks both regionally and abroad.
The group claimed a failed bomb attempt on a U.S.-bound passenger plane in December.
Impoverished Yemen, which is also struggling to curb a rising secessionist movement in the south and to cement a fragile truce with Shiite rebels in the north, has faced international pressure to resolve its domestic conflicts in order to focus on quashing al-Qaeda.