* U.N. says 40 killed in four days of fighting in Aden
* Southern separatists took control of government camps
* Infighting complicates U.N. efforts to end war
* Reports that civilians running out of food, water - U.N.
* Saudi King meets Yemen's beleaguered president
By Mohammed Mukhashef
ADEN, Aug 11 (Reuters) - 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 battling the Iran-aligned Houthi movement.
The infighting, which broke out on Aug. 8 to control the port city which serves as temporary seat of Yemen's Saudi-backed, internationally-recognized government, has killed 40 people and injured 260, the United Nations said.
"It is heart-breaking that during Eid al-Adha (the Muslim holiday) families are mourning the death of their loved ones instead of celebrating together in peace," said Lise Grande, the U.N. Humanitarian Coordinator in Yemen.
"Our main concern right now is to dispatch medical teams to rescue the injured," she said in a statement on Sunday. "We are also very worried by reports that civilians trapped in their homes are running out of food and water."
The Saudi-led, Sunni Muslim coalition said it attacked an area that posed a "direct threat" to the government of President Abd-Rabbu Mansour Hadi, without providing details.
In a sign of support for Yemen's beleaguered president, Saudi Arabia's King Salman met Hadi on Sunday in the Mecca region, on the sidelines of the haj pilgrimage, the state-run Saudi Press Agency reported.
The meeting "discussed efforts to achieve security and stability" in Yemen, it said.
A local official told Reuters the coalition had targeted separatist forces surrounding the nearly empty presidential palace in the Crater district. Hadi is based in the Saudi capital 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.
Several hours after the coalition announcement, there was no indication that STC forces were preparing to leave government military camps they seized on Saturday.
The STC is willing to work with the Saudi-led coalition to ease the crisis and to continue to fight "Iranian expansion," its president Aidaroos al-Zubaidi said in a statement late on Sunday.
He added the council was willing to attend an emergency meeting called by Riyadh on the latest developments. But he did not address the coalition's demand it withdraw.
STC Vice-President Hani Ali Brik, writing on Twitter earlier to mark Eid al-Adha, said while the Council remained committed to the coalition it would "not negotiate under duress". It had earlier agreed to the truce.
The United Arab Emirates-backed separatists, who want to split with the north, have a rival agenda to Hadi's government over the future of Yemen, but 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 in the port city that handles some commercial imports and aid complicates U.N. efforts to end the war that has killed tens of thousands and pushed the country to the brink of famine. Residents said clashes 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 centres.
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 Abu Dhabi and Riyadh, Sunni Muslim allies united against Shi'ite 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 there was 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 the Aden events proved Hadi's government was unfit to rule and called for dialogue with other main powers in Yemen to establish a federation under a "unified national framework".
The United Nations 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 revolt is against corruption.
(Reporting by Nayera Abdallah in Cairo and Reuters team in Aden; Writing by Ghaida Ghantous and Maher Chmaytelli; Editing by Janet Lawrence)
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