The Saudi-led coalition intervened in Aden on Sunday in support of the Yemeni government after southern separatists effectively took over the port city, fracturing the alliance that had been focused on battling the Iran-aligned Houthi movement.
The Sunni Muslim coalition said it attacked an area that posed a "direct threat" to the Saudi-backed government of Abd-Rabbu Mansour Hadi, which is temporarily based in Aden.
It did not specify the site, but a local official told Reuters it had targeted separatist forces surrounding the nearly empty presidential palace in the Crater district. Hadi is based in Riyadh.
"This is only the first operation and will be followed by others ... the Southern Transitional Council (STC) still has a chance to withdraw," Saudi state TV quoted it as saying.
The alliance had threatened military action if the separatists did not quit government military camps they seized in the city on Saturday and halt fighting.
As many as 40 people have been killed and 260 injured in Yemen's southern port city of Aden since Aug. 8, when the latest round of fighting broke out, the office of the United Nations' humanitarian coordinator for the country said in a statement on Sunday, citing preliminary reports.
STC Vice-President Hani Ali Brik, writing in a Twitter post marking a Muslim holiday that began on Sunday, said that while the Council remained committed to the coalition it would "not negotiate under duress". It had earlier agreed to a truce.
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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 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 fighting trapped civilians in their homes with limited water supplies in Aden, the port of which handles some commercial and aid imports. Residents said clashes had ceased on Saturday night.
Coalition member the UAE, which has armed and trained thousands of southern separatists, 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 centers.
The Aden clashes began on Wednesday after the separatists accused an Islamist party allied to Hadi of complicity in a missile attack on a southern forces military parade in Aden.
Analysts said that Abu Dhabi and Riyadh, Sunni Muslim allies united against Shi'ite 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, told Reuters.
"That is the approach again today," she said, but added that 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 on Saturday said that the Aden events proved 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 Hodeidah, north of Aden, 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.