CAIRO—A Saudi-led military coalition fighting in Yemen targeted its own allies with airstrikes on Sunday, a day after southern separatists seized control of the strategic port city of Aden, threatening to fracture the Saudi alliance and open a new front in the four-year-old conflict.

Even before the damage from those strikes had been assessed, the United Nations on Sunday said as many as 40 people had been killed and 260 injured in the previous four days of clashes in Aden that erupted on the eve of one of Islam’s holiest days, Eid al-Adha. Tens of thousands of civilians in the Red Sea city nestled on the tip of the Arabian Peninsula have fled their homes, while many others remain trapped without basic necessities, said UN officials and aid workers.

“It is heartbreaking that during Eid al-Adha families are mourning the death of their loved ones instead of celebrating together in peace,” said Lise Grande, the top UN humanitarian official Yemen, in a statement. “Our main concern right now is to dispatch medical teams to rescue the injured. We are also very worried by reports that civilians trapped in their homes are running out of food and water.”

Yemen, the Middle East’s poorest nation, was already in the grips of what the UN has described as the world’s worst humanitarian crisis.

The seizure of Aden has exposed divides inside the Sunni Muslim coalition led by Saudi Arabia and the United Arab Emirates. Together, they have been battling Iran-allied, Shite rebels known as Houthis to restore Yemen’s internationally recognized government and prevent Tehran from gaining regional influence.

“This weakens the coalition by exposing undeniable cracks beneath the surface,” said Elisabeth Kendall, a Yemen scholar at Oxford University’s Pembroke College. “It is becoming increasingly obvious that the UAE and Saudi Arabia do not share the same end goals in Yemen, even though they share the same overarching goal of pushing back the perceived influence of Iran.”

Rifts have emerged over the past 18 months between the southern separatists, backed by the UAE, and forces aligned with Yemen’s government, backed by Saudi Arabia.

The separatists, who want to split Yemen’s south from its north, have long been suspicious of the Yemeni government, ruled for decades by northerners. The separatists, and the UAE, also disapprove of Yemeni President Abed Rabbo Mansour Hadi’s alliance with Islah, an influential Islamist party. While the Saudis consider Islah vital for rebuilding Yemen, the UAE considers Islah as radicals linked to the Muslim Brotherhood, which is viewed by some regional powers as an extremist organization.

The power struggle for Aden came to a head last week. Clashes engulfed the city’s streets after the separatists accused Islah of playing a role in a missile attack on a military parade that killed dozens of separatist fighters and a prominent commander. Within days, the separatists forces had seized Hadi’s presidential palace and military bases around the city.

The violence was a major blow to Saudi Arabia and its ambition of restoring Yemen’s government. Aden had served as the government’s headquarters for several years, while Sanaa, Yemen’s capital, is under the control of the Houthis, the Iranian-allied rebel group that forced the government from power.

A barrage of statements over the last two days by Saudi government officials reflected the kingdom’s growing alarm at the infighting, framing the violence as a worrying distraction from the campaign against the Houthis.

The Saudi foreign ministry said it had invited the Yemeni government and “all parties involved in the conflict in Aden to hold an urgent meeting in Saudi Arabia to discuss disputes, give priority to prudence and dialogue, and unify ranks.” Col. Turki al-Malki, a spokesman for the Saudi-led coalition, called for a ceasefire to begin early on Sunday and for “all parties to withdraw from positions they have seized.”

Prince Khalid bin Salman, the vice-minister of defence and a brother of Saudi Arabia’s de factor ruler, Crown Prince Mohammed bin Salman, wrote on Twitter that “violence in Aden will create a situation that could be utilized by terrorist organizations like the Houthis, al-Qaida/ISIS, which KSA will never condone.”

Al-Qaida and the Islamic State, also known as Daesh, appear to be taking advantage of the security vacuum in Yemen’s south, said Kendall, who closely monitors both groups. In the first week of August, al-Qaida undertook seven operations in as many days targeting UAE-backed forces, a significant uptick in attacks. And Daesh launched two attacks in Aden, the first time it has targeted the city in more than a year.

Despite the calls for a ceasefire, the Saudi-led coalition opted to send a warning to the separatists with airstrikes late Saturday and early Sunday. While the coalition did not specify where the attacks occurred, separatist officials and aid workers said areas in the Dar Saad and Khormaksar districts, as well as around the presidential palace, were hit.

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“This is only the first operation and will be followed by others … the Southern Transitional Council still has a chance to withdraw,” Saudi state TV quoted the coalition as saying, according to Reuters.

On Sunday, there were no signs that the separatists were leaving the positions they had seized, even though aid agencies and residents reported that fighting had subsided. An uneasy calm prevailed around the city, they said.

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