* Saudis host talks to resolve stand-off in southern Yemen
* Power struggle between nominal allies opens new war front
* Yemen govt wants Saudi ally UAE to stop backing separatists
* Aden crisis complicates U.N. efforts to end ruinous war (Adds comment by Abu Dhabi crown prince)
ADEN, Sept 4 (Reuters) - Yemeni government officials have begun indirect talks with United Arab Emirates-backed southern separatists in the Saudi city of Jeddah to end fighting in Aden and other southern provinces, a Yemeni official said on Wednesday.
The fight for Yemen's south has opened a new front in a multi-faceted war and threatens to further fragment the country, complicating efforts to end a conflict that has killed tens of thousands and pushed millions towards famine.
Government and separatist forces are both part of a Saudi-led coalition that intervened in Yemen in March 2015 against the Iran-aligned Houthi group that had removed President Abd-Rabbu Mansour Hadi from power in the capital Sanaa the previous year.
But the separatists, who seek to restore the former South Yemen republic, have seized control of the southern port of Aden and declared Sanaa government authorities unwanted in the south.
"Indirect talks have started between the government and the STC (Southern Transitional Council separatists) via the Saudi side," a senior Yemeni official, who declined to be named, told Reuters. "The situation is very difficult and complicated but we hope to achieve some progress."
Last week the UAE, Saudi Arabia's main coalition partner, carried out air strikes on government forces to support the separatists as STC forces recaptured control of Aden, forcing a government retreat.
A senior Emirati official said earlier that the Gulf state was confident that the Jeddah meeting would succeed.
"We are looking with confidence and optimism at the success of the Jeddah meeting between Yemen's government and the STC, and unity against the Houthi coup," Minister of State for Foreign Affairs Anwar Gargash said in a Twitter post.
CALL FOR SUMMIT
Saudi Arabia has called for a summit meeting in Jeddah to defuse the stand-off. Aidarous al-Zubaidi, leader of the STC, and Yemeni government officials arrived there earlier this week.
But Hadi's government on Wednesday again criticised the UAE for backing the separatists, and stressed it was not in direct talks with the STC. The UAE in June withdrew some of its own forces and hardware from the region but maintains influence via tens of thousands of southern fighters it has armed and trained.
"There are no (direct) talks of any kind to this point between the government and the STC," Yemen's state news agency SABA quoted government spokesman Rajeh Badi as saying.
The foreign ministry said in a Twitter post that a "serious and transparent stand must be taken against the UAE's deviation from the coalition" for any dialogue to be successful.
Saudi Arabia has been struggling to preserve the coalition since Hadi's government and the STC turned on each other, exposing a rift between Riyadh and Abu Dhabi.
Both countries have said their alliance remains strong. The UAE's de facto ruler Abu Dhabi Crown Prince Sheikh Mohammed bin Zayed al-Nahyan said their futures were bound together "in the trench".
"The goal that unites is the security of Saudi Arabia and the UAE, and regional stability," he tweeted on Wednesday.
The cracks in the alliance risk scrambling United Nations efforts to bring an end to a war largely seen as a proxy struggle between Saudi Arabia and Iran for regional dominance.
Yemen's south is the only territory seized by the coalition in the war, an area where the Saudis hold less sway. The Houthis, who have stepped up attacks on Saudi cities, hold most urban centres including Sanaa and the main port of Hodeidah.
(Reporting by Reuters team in Yemen with additional reporting by Alaa Swilam in Cairo Writing by Aziz El Yaakoubi Editing by Mark Heinrich and John Stonestreet)