(AFP) -- Saudi and Emirati envoys shuttled between Yemeni Hadi forces and besieging southern separatists in second city Aden Thursday in a bid to end a tense standoff after days of deadly infighting.

The Sunday assault on the embattled former government's headquarters by its former allies has opened up a new front in the devastating civil war that has created what the United Nations says is the world's worst humanitarian crisis.

Saudi Arabia and the United Arab Emirates are the two major contributors to a military coalition that has backed Abedrabbo Mansour Hadi since he fled into exile in 2015.

But they have struggled to keep together the disparate alliance supporting him against Shiite Huthi fighters who control the capital Sanaa and much of the north.

South Yemen was an independent country until union with the north in 1990 and Hadi has relied heavily on militia that support its restoration.

Many of them have been recruited into special forces units trained by the UAE to fight Al-Qaeda, which has a large presence in parts of the south.

On Wednesday, those forces deployed across Aden bringing a lull in the deadly clashes that had forced a halt to the distribution of desperately needed relief supplies for days.

Saudi Arabia and the UAE have not abandoned their support for Hadi, who lives in exile in Riyadh, but they have singularly failed to intervene militarily in support of Prime Minister Ahmed bin Dagher and other ministers who are holed up under siege in the presidential palace in Aden.

The two governments' envoys "met with all concerned parties, stressing the need to abide by the ceasefire ... and refocus efforts on the front lines against the Huthis", the UAE's official WAM news agency reported.

The UAE has close ties to separatist Hani bin Breik, a leader in the Southern Transitional Council to which many of the forces now in control of Aden are loyal.

Both it and Saudi Arabia have urged Hadi's government to heed the separatists' grievances and called on all sides to show restraint.

At least 38 people have been killed and 222 wounded in Aden since Sunday, according to the International Committee of the Red Cross.

The UN agency for humanitarian affairs reported a "cautious calm" in the city late on Wednesday but said aid ships were still unable to dock.

Even before the latest fighting, Yemen already faced the world's most serious humanitarian crisis, with some 8.4 million of its 22.2 million population at risk of famine, according to the UN.