BEIRUT, Lebanon — Deadly fighting between government forces and separatists escalated in the southern Yemeni city of Aden on Friday, driving a wedge through their fragile alliance and threatening to exacerbate the country’s multisided civil war.

The Yemeni national airline rerouted flights from Aden to avoid the violence, and residents reported gunmen in the streets, blasts from heavy weapons and acute water shortages. At least 25 people have been killed since the fighting began on Wednesday, with four times as many wounded, said Salem al-Shabahi, a local health official.

The clashes in Aden, where the internationally recognized government is now based, are akin to a small civil war inside the larger one that split the country nearly five years ago, killing tens of thousands of people and plunging Yemen into what the United Nations has called the world’s worst humanitarian crisis.

The fighting that broke out on Wednesday has been between two groups that are nominally allies: the separatists, who aspire to make southern Yemen independent, and the forces loyal to Yemen’s exiled president, Abdu Rabbu Mansour Hadi. Throughout the war, the two groups have been part of a Sunni coalition fighting the Houthi rebels, who practice an offshoot of Shiite Islam and have taken over the country’s northwest.