Several people were killed after clashes broke out between United Arab Emirates-backed southern separatists and forces loyal to the Saudi-based government, near the port city of Aden on Sunday.
Gunfire and explosions rang out in the city, as both sides fought for control of strategic military bases and government buildings.
Tensions had been simmering since the Southern Transitional Council, supported by the UAE, demanded the Saudi-backed government to resign en masse. The deadline for President Abd-Rabbu Mansour Hadi to dismiss the cabinet expired on Saturday.
Factional infighting has threatened the previously united war effort against Houthi rebels, who control the north of the country.