A Saudi-led coalition launched Sunday a strike against Yemen's southern separatists after they seized the presidential palace in second city Aden in deadly fighting that threatened to push the war-ravaged nation deeper into turmoil.
The seizure, decried by the Riyadh-backed Yemeni government as a UAE-supported coup, reflects deep divisions between secessionists and loyalist forces, both of whom have fought Shiite Huthi rebels.
"The coalition targeted an area that poses a direct threat to one of the important sites of the legitimate government," a statement said, calling on the separatist Southern Transitional Council (STC) to withdraw from positions seized in Aden or face further attacks.
It did not specify the target but residents in Aden told AFP it was an air strike against separatist camps in the city.
Since the fighting flared on Thursday, around 40 people have been killed and 260 others including civilians wounded, according to the UN.
"It is heart-breaking that during Eid al-Adha, families are mourning the death of their loved ones instead of celebrating together in peace and harmony," said UN Humanitarian Coordinator in Yemen Lise Grande.
Riyadh-based Yemeni President Abedrabbo Mansour Hadi is backed by the coalition -- led by Saudi Arabia and its ally the United Arab Emirates -- that is battling the Iran-aligned Huthis.
But another force in the anti-Huthi coalition -- the UAE-trained Security Belt Force -- has since Wednesday been battling loyalists in Aden, the temporary base of Hadi's government.
The Security Belt Force is dominated by fighters who back the STC, which seeks to restore south Yemen as an independent state as it was from 1967-1990.
The International Crisis Group think tank warned that the Aden clashes "threaten to tip southern Yemen into a civil war within a civil war."
"Such a conflict would deepen what is already the world's worst humanitarian crisis," it said.
The coalition called for an "immediate ceasefire" and the Saudi foreign ministry has demanded an "urgent meeting" between the warring parties.
Both the Yemeni government and separatists said Sunday they backed Riyadh's call for dialogue and a suspension of fighting.
But in a sermon to mark the start of the Muslim Eid al-Adha festival, STC vice president Hani bin Breik said his group will not "negotiate under threat".
Saudi Deputy Defence Minister Khalid bin Salman also called for an "immediate cessation of hostilities and withdrawal from all forcefully occupied locations in Aden".
A Security Belt official told AFP Saturday the force had seized the presidential palace -- largely symbolic, due to Hadi's absence -- without a fight.
"Two hundred soldiers from the Presidential Guard were given safe passage out of the palace," the official said.
Yemen's government earlier blamed the STC and the UAE for staging a "coup" against it.
The foreign ministry demanded "the UAE halt its material support and withdraw its military support, immediately and fully, from the groups that have rebelled against the state".
The STC's spokesman said the council was working to restore the water network, damaged in the fighting.
Ties between the Security Belt and Hadi loyalists have been strained for years, and this week was not the first time they have engaged in armed clashes.
They fought a three-day battle in January 2018 that killed 38 people and wounded 222 others after the government prevented a rally by separatists.
The Security Belt has accused Hadi's backers of allowing Islamists into their ranks and of supporting the Muslim Brotherhood.
The Saudi-led coalition intervened in Yemen in 2015 to back the government against the Huthis -- backed by Riyadh's regional rival Iran -- who control large swathes of Yemen including the capital Sanaa.
UAE Foreign Minister Abdullah bin Zayed Al-Nahyan said Abu Dhabi was "exerting all efforts to de-escalate the situation", Emirati state media reported.
The two camps should focus their efforts on fighting the Huthis instead of each other, he said.
The latest violence erupted on Wednesday during the funeral of a senior Security Belt commander killed earlier this month in a drone and missile attack on a training camp west of Aden.
The commander was among 36 people killed -- many of them newly graduated cadets -- in the aerial attack, claimed by the Huthis.
The UN human rights office later accused the Security Belt force of "reportedly carrying out and enabling retaliatory attacks against civilians" from northern Yemen.
Since 2015, fighting between the Huthis and Yemeni loyalists backed by the Saudi-led coalition has killed tens of thousands of people, mostly civilians, aid agencies say.
Aden, Yemen | AFP