AL MUKALLA, Yemen — A simmering split within the Saudi-led coalition that has been fighting rebels in Yemen since 2015 exploded into deadly combat this week, paralyzing the southern city of Aden, the government’s temporary seat of power.

The fighting, which began Sunday, has pit allies armed with heavy weapons against each other and further complicated prospects for a resolution of the war in Yemen, the Middle East’s poorest country and home to one of the world’s worst man-made humanitarian crises.

The coalition includes forces loyal to the Saudi-backed president, Abdu Rabbu Mansour Hadi, and a faction supported by the United Arab Emirates, known as the Southern Transitional Council. The council has accused Mr. Hadi’s subordinates of corruption and incompetence, and has advocated the revival of a separate state in southern Yemen, which merged with the north in 1990.

Last week, the council gave Mr. Hadi an ultimatum to dismiss the cabinet, and as the deadline neared, the forces of the two sides started shooting at each other on Sunday with tanks, artillery and automatic weapons.