Yemeni separatists backed by the United Arab Emirates began withdrawing Sunday from positions they seized from the internationally-recognised government in the southern port city of Aden.
Both the southern separatists and the government forces are ostensibly allies in the Saudi-led military coalition that's been battling the Houthi rebels in northern Yemen since 2015.
But a major rift in the coalition was exposed during the four days of fighting for control of Aden, as the UAE-backed Southern Transitional Council wrested the city from government forces. More than 70 people were killed in the clashes.
The UAE is the dominant force in Yemen's south, where it has an estimated 90,000 allied militiamen and has long been at odds with the government, which is largely based in Saudi Arabia.
Saudi Arabia views the Houthi rebels in Yemen's north as a major national security threat, in part because the Houthis have launched numerous cross-border missile attacks targeting the Saudi capital and other cities.
The Saudi-led coalition on Sunday intervened in Aden in support of the Yemeni government after southern separatists effectively took over the southern port city, fracturing the alliance that had been focused on battling the Iran-aligned Houthi movement.
Dozens of people have been killed and 260 injured in the southern port city since August 8, when the latest round of fighting broke out, the office of the United Nations' humanitarian coordinator for the country said in a statement.
The Western-backed coalition said it attacked an area that poses a "direct threat" to the Saudi-backed government of Abd Rabbu Mansour Hadi, which is temporarily based in Aden. It did not specify the target or military action taken.
The alliance had threatened military action if the separatists did not quit the government military camps they captured on Saturday, which later led to the separatists taking a turn to withdraw from positions they seized from the government.
The United Arab Emirates-backed separatists have a rival agenda to Hadi's government over the future of Yemen, but they have been a key part of the coalition that intervened in the Arabian Peninsula nation in 2015 against the Houthis after the group ousted Hadi from power in the capital Sanaa in late 2014.
The violence complicates the United Nations' efforts to end the war that has killed tens of thousands and pushed the long-impoverished country to the brink of famine.
The STC forces had seized the government military bases and surrounded the nearly empty presidential palace after four days of clashes.
After the coalition warning, the STC had agreed to the ceasefire. Residents said there had been no clashes between the separatists and government forces since the previous night.
The UAE, a coalition member, which has armed and trained thousands of southern separatists, had urged calm. Riyadh said it would host an emergency meeting aimed at restoring order. Hadi's government has asked Abu Dhabi to stop backing southern forces.
Setback for coalition
The infighting is a serious setback for the coalition in its more than four-year campaign to break the grip of the Houthis, who control Sanaa and most urban centres. Hadi's government holds Aden although the president is based in Riyadh.
The Aden clashes began on Wednesday after the separatists accused a religious party allied to Hadi of complicity in a missile attack on a southern forces military parade in Aden.
Analysts said Abu Dhabi and Riyadh, allies united against foe Iran, would work together to contain the crisis even though the UAE in June scaled down its military presence in Yemen as Western pressure mounted to end the war.
"The UAE and Saudi Arabia have allied with distinct Yemeni partners ... Yet to this point in the conflict, Abu Dhabi and Riyadh have worked to maintain a relative detente between competing interests in the south," Elizabeth Dickinson, senior analyst at International Crisis Group said.
"That is the approach again today," she said, but added there was real concern that the situation could deteriorate into "a civil war within a civil war".
The war has revived old strains between north and south Yemen, formerly separate countries that united into a single state in 1990 under slain former President Ali Abdullah Saleh.
The Houthis' deputy foreign minister said on Saturday that the Aden events proved that Hadi's government was unfit to rule and called for a dialogue with other main powers in Yemen to establish a federation under a "unified national framework".
The UN is trying to salvage a stalled peace deal in the main port city of Hudaida to pave the way for peace talks at a time of heightened tensions after the Houthis stepped up missile and drone attacks on Saudi cities.
The Yemen conflict is widely seen in the region as a proxy war between Saudi Arabia and Iran. The Houthis deny being puppets of Iran and say their revolution is against corruption.