Mr. Hadi was not in Aden when the clashes between his loyalists and the Southern Transitional Council separatists began. He was believed to be in Saudi Arabia, where he has spent much of his time for the past three years.
After two days of what witnesses and local news reports called fierce shelling and shooting, Mr. Hadi’s forces in Aden crumbled on Tuesday, and the separatists seized the area around the presidential palace where his prime minister and other officials had been staying. Their whereabouts as of Tuesday night were unclear.
The official Saba news agency in Yemen quoted local health officials as saying that as many as 21 people had been killed and 290 wounded since Sunday.
Hani bin Bourek, a senior member of the Southern Transitional Council, said the separatists had pulled their forces back after mediation by other members of the Saudi-led coalition.
In an interview with France 24’s Arabic-language channel, the president of the council, Aidarous al-Zubaidi, said his fighters were still “engaged in a mission with the Saudi-led coalition.”
Whether the council’s rift with Mr. Hadi’s forces could be mended was uncertain at best.
Saudi Arabia’s official press agency, quoting a coalition statement, said that the coalition had been “watching with regret all over the past two days that all parties have not responded to the calls for calm” and that it had requested “all parties to speed up the cessation of all clashes immediately.”
International aid groups and the United Nations, which have been struggling to deliver emergency aid to Yemeni civilians, appealed for calm in Aden, which had been a relatively stable area until shooting and artillery blasts erupted on Sunday.
“The fighting in Aden makes it impossible for us to carry out our lifesaving work,” Tamer Kirolos, the director of Save the Children’s operations in Yemen, said in a statement. “Our staff are forced to shelter at home and in bunkers while gun battles rage outside.”
A spokesman in New York for the United Nations, Stéphane Dujarric, said the organization’s relief officials were “extremely concerned by the violence that we’ve seen over the last couple of days.”
The lack of any progress on political negotiations, Mr. Dujarric said, “only piles onto the misery of the Yemeni people.”
The war has left more than 10,000 people dead, displaced at least two million and contributed to an acute hunger crisis that has put much of the population of roughly 27 million close to famine.
Efforts by a succession of United Nations mediators to hold negotiations on the conflict have ended in failure. Less than two weeks ago, Ismail Ould Cheikh Ahmed, a veteran diplomat from Mauritania, announced he was vacating the mediator post in February.