CAIRO —   A declaration of self-rule by the most influential Yemeni southern separatist group has triggered concerns that Yemen's conflict could escalate just as the United Nations is seeking a nationwide cease-fire to prevent the spread of the novel coronavirus.

The United Arab Emirates, the separatists’ main backer, said Monday that it opposes the declaration and urged the group to honor a peace deal it signed last year with the internationally recognized government affirming shared control of the south. Saudi Arabia — which, along with the UAE, leads a regional Sunni Muslim coalition fighting in Yemen — called for the separatists to reverse their move, describing it as “an escalatory action.”

The United Nations also voiced concern, urging the feuding sides to adhere to the peace deal, known as the Riyadh Agreement. Accusations of delays in implementing the truce appear to have prompted the Southern Transitional Council’s decision over the weekend to declare self-rule.

The tensions were the latest jolt to Yemen’s south, after at least 14 people, including five children, were killed in flash floods last week in the Red Sea city of Aden, the temporary capital of the internationally recognized Yemeni government. The country, the Arab world’s poorest, is already in the grip of the world’s most severe humanitarian crisis after more than five years of conflict. This month the first case of covid-19, the disease caused by the novel coronavirus, was detected, sparking fears that the disease could quickly spread at a time when Yemen is facing deep cuts in humanitarian aid funding.

“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,” Martin Griffiths, the U.N. special envoy for Yemen, said in a statement. “Now, more than ever, all political actors must cooperate in good faith, refrain from taking escalatory actions, and put the interests of Yemenis first.”

The fresh animosities are a reminder of the multiple wars suffocating Yemen. While the primary, and best-known, conflict pits the Saudi-led coalition against northern rebels known as Houthis, the battle for the strategic port city of Aden and other areas in the south involves two allies within the coalition.

The UAE supports the separatist Southern Transitional Council, while Saudi Arabia backs the internationally recognized Yemeni government of President Abed Rabbo Mansour Hadi. All are against the Iran-aligned Houthis, but rifts inside the coalition have been simmering for more than two years.

The STC, which favors splitting Yemen’s south from its north, has long tangled with the Yemeni government, ruled for decades by northerners. The group and the UAE disapprove of Hadi’s alliance with al-Islah, an influential Islamist party. The Saudis view al-Islah as part of Yemen’s political fabric. But the UAE and the STC oppose any role for al-Islah because of its links to the Muslim Brotherhood, a regional political Islamist movement that the Emiratis and other Arab rulers have labeled terrorists.

Last August, STC fighters seized control of Aden after four days of clashes that killed as many as 40 people, injured 260 and forced tens of thousands of civilians to flee the city, nestled on the tip of the Arabian Peninsula. That prompted the coalition to target its own allies with airstrikes.

In November, the STC and the government signed a peace agreement in the Saudi capital, Riyadh. It was hailed by Saudi Arabia, the UAE and Western powers as paving the way for a broader political solution that would end Yemen’s civil war and humanitarian crisis, which has left millions on the edge of starvation and vulnerable to outbreaks of disease.

The deal called in particular for a government composed of equal numbers of southerners and northerners, for the separatist forces to come under the control of the government, and for the STC to return all government buildings it had seized.

But the Riyadh deal did not address southerners’ underlying grievances or their main issue of secession, said analysts. Deadlines repeatedly slipped for military integration and creating a power-sharing government of technocrats. Each side blamed the other.

Meanwhile, international attention was focused on a Saudi cease-fire with the Houthis and the emergence of the coronavirus. Regional powers were distracted by the start of the Muslim fasting month of Ramadan on Friday.

“This was an opportune moment for the STC to take a stand,” said Elisabeth Kendall, a Yemen scholar at Oxford University. The flooding, she added, also set off “a political point-scoring match between the government and the STC, both of whom hailed the poor response as evidence of the other party’s inability to govern.”

Over the weekend, it became clear that the mistrust had deepened once again.

The STC vice president, Hani Ali Brik, sent a tweet Sunday morning accusing the Hadi government of setting up obstacles to the deal and of corruption and mismanagement. Hours earlier, the separatists announced emergency rule in Aden and other southern provinces. They also declared their intent to take control of the port, the airport, the central bank and other government areas.

Officials in three southern provinces — Shabwa, Hadhramaut and Socotra — rejected the self-rule declaration. Yemen’s foreign minister said the announcement was “a resumption of its armed insurgency” and warned of “dangerous and catastrophic consequences.”

The self-rule move comes as the Hadi government has been weakened by significant Houthi military gains and signs that the Saudis are looking for a way to exit the war. “This is just one more blow to its credibility,” Kendall said of the government.

It could also hurt peace prospects. The declaration “makes a cease-fire and political settlement harder,” said Peter Salisbury, the International Crisis Group’s senior analyst for Yemen. The Riyadh deal opened a path for a more representative Yemeni government, giving it more credibility to negotiate with the Houthis.

“If the deal collapses, that is taken off the table,” Salisbury said. “Secondly, the STC are one of the groups fighting the Houthis on the ground. If they aren’t part of a cease-fire deal, then the Houthis can argue there is no cease-fire.”

“The short-term winners are of course the Houthis, since any resurgence of rifts in the coalition distracts energy and resources away from the war against them,” Kendall said.