A Saudi-led coalition mired in a years-long war in Yemen has urged Emirati-backed southern separatists to honour terms of a Riyadh peace deal and share control of Aden with the country's internationally recognised government.

The statement by Saudi Arabia comes after the separatists' Southern Transitional Council again claimed sole control of Aden, a Red Sea port that serves as the seat of the internationally recognised government as Yemen's Shia rebels, known as Houthis, hold the country's capital, Sanaa.

The council's decision adds yet more complexity to the war in Yemen that has pushed the Arab world's poorest nation to the brink of famine and killed more than 100,000 people.

The Saudi statement urged the council to return to the terms of the November 2019 Riyadh agreement, which ended fighting after the council seized control of Aden just a few months earlier.

That deal had called for both sides to remove heavy military equipment from Yemeni cities under their control and form a unity government that included equal representation.

But that deal had yet to be implemented as the war continued, massive floods struck Aden and Yemen faces the threat of the coronavirus pandemic.

The Saudi-led coalition urged the council to honour the deal "to unite Yemeni ranks, restore state institutions and combat the scourge of terrorism".

The deal "entails forming a competent government that operate from the interim capital Aden to tackle economic and developmental challenges, in light of natural disasters such as floods, fears of the Covid-19 pandemic outbreak and work to provide services to the brotherly people of Yemen".

The statement did not say what Saudi Arabia would do if the council refused.

The kingdom, which is focused on the coronavirus outbreak there, declared a unilateral ceasefire with the Houthis in April that was later extended through the holy Muslim month of Ramadan.

The Southern Transitional Council had been the on-the-ground allies of the United Arab Emirates, once Saudi Arabia's main partner in the war that subsequently withdrew from the conflict.

The council's backers often fly the flag of former Communist South Yemen and have pushed to again split the country into two like it was from 1967 to 1990.

Emirati Minister of State for Foreign Affairs Anwar Gargash said on Twitter that his nation had "absolute confidence" in Saudi Arabia's desire to formalise the power-sharing agreement.

"Frustration over delaying implementation of the agreement should not be a reason to unilaterally change the status quo," he wrote.

The Houthis in 2014 overran major parts of northern Yemen, including Sanaa, pushing out the internationally recognised government.

The Saudi-led coalition entered the conflict in 2015, backing the internationally recognised government in 2015.

In Brussels, European Commission spokesman Peter Stano told reporters that the EU has taken note of the developments in southern Yemen, which he said undermine the Riyadh agreement, a key to de-escalation.

"What Yemen needs right now is peace," he said, calling on the separatists "to implement the provisions that were agreed under the auspices of Saudi Arabia in this agreement."