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.
In spring 2015, Saudi Arabia, the United Arab Emirates and other Arab countries launched an air campaign aimed at pushing back the Houthis and restoring Mr. Hadi’s government. They also each supported forces on the ground, with the Saudis backing Mr. Hadi’s troops and the Emiratis backing the secessionists.
Tensions between those two groups have flared up in the past, but this week’s violence was the most intense since the Emirates announced last month that it was drawing down its military presence in Yemen.
Analysts studying the conflict said it was too soon to tell whether the fighting between Saudi- and Emirati-backed forces signaled a wider shattering of their alliance. But they worried that if the fighting was not contained, it could spread, further complicating the Yemeni conflict and the efforts to resolve it.
“The knock-on effect of this could be very big if it is allowed to spiral,” said Peter Salisbury, senior analyst for Yemen at the International Crisis Group. “This basically creates a civil war within a civil war.”
The United Nations has been trying for years to end the conflict through peace talks, with little progress. Mr. Salisbury said a deeper split in the country’s south could make that challenge even greater by giving credence to the Houthis’ argument that Mr. Hadi lacks sufficient control to enforce a peace deal. Mr. Hadi, 73, and many of his ministers spend most of their time in Saudi Arabia, shielded from the conflict tearing their country apart.
Yemen’s war has fueled a humanitarian catastrophe, with cholera outbreaks, widespread hunger and mass malnutrition. The fighting in Aden has worsened the situation there, blocking civilians from fleeing and interrupting the delivery of drinking water during the hottest part of the year. One resident said people were collecting water dripping from air-conditioners to drink.
“We do not know who is fighting who in the street or who is has the upper hand,” said another resident, Ahmed Salem.