The Saudi-led military coalition on Monday rejected the Southern Transitional Council's declaration of self-rule over Yemen's south and called for a return to an agreement reached in Riyadh to end months of tensions.
The STC's move complicates the ongoing fight by the coalition and the internationally recognised government against Houthi rebels who control much of the north.
The southern group signed a power-sharing deal in Riyadh last November that ended a battle for the south that caused gunfights in the second-largest city of Aden in August.
"Following the surprising announcement of a state of emergency by the Southern Transitional Council, we re-emphasise the need to promptly implement the Riyadh Agreement," the coalition said, according to tweets from the official Saudi Press Agency.
"The Coalition demands an end to any escalatory actions and calls for return to the agreement by the participating parties."
Saudi deputy defence minister, Prince Khalid bin Salman, also called on the STC to return to the agreement.
"During these challenging global times, leaders in Aden must realise their historic responsibility to keep the Yemeni people safe, and more violence will not achieve that. The Riyadh agreement, which is backed by the international community, must be implemented," he said.
Dr Anwar Gargash, the UAE's Minister of State for Foreign Affairs, tweeted on Monday to say his country backs the Saudi efforts to end the dispute and say that frustrations over implementing the deal are not a reason to unilaterally change the situation.
“The Statement of the Alliance to Support Legitimacy comes from a clear concern for Yemen and the Riyadh Agreement, whose full implementation represents a basis for political work in the next stage,” Dr Gargash tweeted.
“Frustration with the delay in implementing the agreement should not be a reason to change the situation from one side and our confidence in the sister Saudi Arabia’s commitment to implement the Riyadh Agreement is absolute.”
He said implementing the agreement and “adhering to its spirit” is the “cornerstone of the regional and international vision of the political solution in Yemen, and the statement of the coalition emphasises the need for all parties to implement the agreement in order to ensure the coherence of the political and military situation and pave the way for the next stage.”
On late Saturday, the STC declared self-rule in southern Yemen, accusing the government of failing to perform its duties and of "conspiring" against the southern cause.
The government has condemned the move and said the separatists – who have long agitated for independence in the south – would be responsible for the "catastrophic and dangerous" outcome.
The breakdown between the long-time allies comes as the coalition has extended a unilateral ceasefire to allow the country prepare for the coronavirus pandemic – an olive branch rejected by the Houthis.
Compounding the country's troubles, at least 21 people were killed in flash flooding this month, with Aden's streets submerged and homes destroyed.
The United Nations said on Sunday that more than 100,000 people across Yemen have been affected by the torrential rains, which had 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.
"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 said.
"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.
"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," Adel Al Jubeir, the Saudi Minister of State for Foreign Affairs, said in a tweet.
"We reject any hostilities that will jeopardise the safety and stability of Yemen," he said.
UN Envoy to Yemen Martin Griffiths also expressed concern over the Saturday declaration by the STC.
“The latest turn of events is disappointing, especially as the city of Aden and other areas in the south have yet to recover from flooding and are facing the risk of Covid-19,” Mr Griffiths said.
He called the Riyadh Agreement to be implemented quickly with the support of the coalition. He added that the success of this agreement should deliver benefits to the people in the south, particularly in terms of improved public services and security.
“Now, more than ever, all political actors must cooperate in good faith, refrain from taking escalatory actions, and put the interests of Yemenis first,” Mr Griffiths said. “The Riyadh Agreement provides for the participation of the STC in consultations on the final political solution to end the conflict in Yemen and serving the interests of Yemenis nation-wide.”
The STC has been a crucial ally in the long war against the Houthis, providing a large number of fighters and brigades on the front lines but the secessionists believe the south should be an independent state – as it was before unification in 1990.
On Sunday, Aden residents reported heavy deployments of STC forces in the city and a separatist source told AFP they had set up checkpoints "at all government facilities, including the central bank and port of Aden".
Hussam Radman, a research fellow for the independent Sanaa Centre 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.