Residents of the southern city of Aden reported heavy deployments of STC forces, and a separatist source told AFP they had set up checkpoints 'at all government facilities, including the central bank and port of Aden'
The Saudi-led military coalition in war-torn Yemen on Monday rejected a declaration of self-rule by separatists in the country's south and demanded "an end to any escalatory actions."
The breakaway declaration made Sunday threatens to reignite a "war within a war" in the Arabian Peninsula's poorest nation that is already gripped by what the United Nations calls the world's worst humanitarian disaster.
The secessionists' move significantly complicates the country's five-year-old wider conflict, fought by the Saudi-led coalition and Yemen's internationally recognized government against Iran-backed Huthi rebels who control much of the north.
Yemen's separatists in the south, which used to be a separate country, have repeatedly agitated to break away again -- a campaign that was temporarily put to rest with a power-sharing deal signed in Riyadh last November.
But on Sunday the Southern Transitional Council (STC) declared self-rule in southern Yemen, accusing the government of failing to perform its duties and of "conspiring" against the southern cause.
Residents of the southern city of Aden reported heavy deployments of STC forces, and a separatist source told AFP they had set up checkpoints "at all government facilities, including the central bank and port of Aden."
The Yemeni government condemned the move and warned it could lead to a "catastrophic and dangerous" outcome.
The coalition said, according to Saudi Press Agency tweets, that "we re-emphasize the need to promptly implement the Riyadh Agreement."
"The coalition demands an end to any escalatory actions and calls for return to the agreement by the participating parties."
Key coalition partner the United Arab Emirates, which has supported the STC, also stressed the importance of abiding by the Riyadh pact.
"Frustration over delay in implementing the agreement is not a reason to unilaterally change the situation," UAE's minister of state for foreign affairs Anwar Gargash tweeted.
"We have full confidence in Saudi Arabia's keenness to implement the agreement."
Virus, floods, cholera
The breakdown between the one-time allies comes as the coalition has extended a unilateral ceasefire aimed at fending off the coronavirus pandemic -- an olive branch that has been rejected by the Huthis.
Compounding the country's troubles, at least 21 people have been killed in flash floods this month, which left Aden's streets submerged and homes destroyed.
The United Nations said Sunday that more than 100,000 people across Yemen have been affected by the torrential rains which damaged roads, bridges and the electricity grid and contaminated water supplies.
"Countless families have lost everything," Lise Grande, the UN's Humanitarian Coordinator for Yemen, said in statement.
"This tragedy comes on top of the Covid-19 crisis, which comes on top of the pre-famine last year, which came on top of the worst cholera outbreak in modern history," she added.
"The solution is clear. The parties to the conflict need to find the courage to stop fighting and start negotiating."
The Riyadh pact on power-sharing for the south had been hailed as averting the complete break-up of the country, but with a lack of implementation, observers have said it is effectively defunct.
Cracks emerged soon after it was signed, with complaints over food shortages in the south, a sharp depreciation of the currency and a lack of funds to pay public sector employees.
Saudi state minister of foreign affairs Adel al-Jubeir said in a tweet that "we in (Saudi Arabia) and UAE strongly believe that the internationally backed Riyadh agreement has guaranteed an opportunity for the brotherly Yemeni people to live in peace."
"We reject any hostilities that will jeopardize the safety and stability of Yemen."
While the government and the STC have been technically allies in the long war against the Huthis, the secessionists believe the south should be an independent state -- as it was before unification in 1990.
Hussam Radman, a research fellow for the Sanaa Center for Strategic Studies, said the separatists were already in control of the military and security in Aden, where they have popular support.
"But with this declaration, it will become responsible for the administrative side in the provisional capital that has witnessed an unprecedented decline lately" in the provision of services and economic performance, he told AFP.