By Stephen Kalin and Ghaida Ghantous
RIYADH/DUBAI (Reuters) - The southern separatists' takeover of Aden, the interim seat of Yemen's government, could leave Saudi Arabia struggling to hold together a military coalition fighting the Iran-aligned Houthis.
It also risks fragmenting southern Yemen as the United Nations struggles to restart talks to end the 4-1/2-year war that has pushed millions to the brink of famine.
WHAT HAS HAPPENED IN YEMEN?
Separatists, who want to split from the north and are backed by the United Arab Emirates, effectively seized Aden by taking over the government's military bases on Saturday after they accused a party allied to President Abd-Rabbu Mansour Hadi of complicity in a Houthi missile attack on their forces.
The Saudi-led coalition backing Yemen's government hit back on Sunday, saying it attacked one target, after threatening to act if southern forces do not cease fighting.
The two sides had been nominally allied in the coalition fighting the Houthis, who ousted Hadi's government from the capital Sanaa in 2014, but they have rival agendas.
WHAT DOES THIS MEAN FOR THE COALITION?
It makes it harder for Saudi Arabia to weaken the grip of the Houthis, who hold Sanaa and most urban centres.
The Western-backed, Sunni Muslim coalition intervened in Yemen against the Houthis in 2015. The conflict is widely seen as a proxy war between Saudi Arabia and Iran.
The Houthis have no traction in the south, where the UAE has armed and trained 90,000 Yemeni troops drawn from southern separatists and coastal plains fighters.
But the Southern Transitional Council that leads the separatists may not have broad support outside Aden. Its move risks igniting infighting in the south and emboldening militant groups like al Qaeda, among Yemen's many destabilising forces.
HOW DID IT REACH THIS POINT?
There is no love lost between the separatists and Hadi's government, which they accuse of mismanagement and corruption.
The war has revived old strains between north and south Yemen, formerly separate countries that united into a single state in 1990.
This is not the first separatist uprising. They seized Aden in January 2018. Riyadh and Abu Dhabi helped end that standoff.
The UAE has asked U.N. envoy Martin Griffiths to exert pressure on both sides. Riyadh said it would host an emergency summit of the parties to restore order.
"Recruiting separate militias across the south ... was always playing with fire. It’s a bit rich now of the UAE to say the U.N. special envoy needs to sort it out," said Elisabeth Kendall of Oxford University.
IS THE SAUDI-UAE ALLIANCE IN YEMEN BROKEN?
The coalition is fractured but not broken. Analysts say the UAE is unlikely to recommit troops but will support Riyadh, with which it is working to contain Shi'ite Iran.
"This could be a turning point but it will be papered over by the leaderships in the way that it always is - but papering over bigger and bigger cracks now so the paper is thinner and thinner," Kendall said.
The UAE said it scaled down its presence in Yemen due to a holding truce in the main port of Hodeidah, which became the focus of the war last year when the coalition tried to seize it.
Diplomats say it was because the UAE accepted there could be no military solution due to global criticism of coalition air strikes that have killed civilians and the humanitarian crisis.
Western pressure to end the war that has killed tens of thousands, and heightened U.S.-Iran tensions, which risk triggering a war in the Gulf, added impetus to the decision.
WHAT CAN THE U.N. DO?
For now shuttle diplomacy. Griffiths had been trying to salvage a stalled troop withdrawal deal agreed by the Houthis and Hadi's government at December peace talks in Sweden.
He is also trying to calm tension between the Houthis and Saudi Arabia after the movement stepped up missile and drone attacks on Saudi cities in recent months.
But if any broader political talks on a transitional ruling body materialise, they would have to include more of Yemen's fractious parties, including southern separatists.
(Reporting by Stephen Kalin and Ghaida Ghantous; Writing by Ghaida Ghantous; Editing by Janet Lawrence)