The latest inter-ethnic violence to rock Sudan's restive Darfur region had killed nearly 140 people by Monday, tribal leaders said just weeks after a peacekeeping mission ended operations.

Clashes left 55 people dead in South Darfur on Monday, after a separate round of violence in West Darfur reportedly killed 83 and displaced tens of thousands over the weekend.

It was the worst violence reported since the signing of a peace agreement in October, which observers had hoped would end years of war.

The bloodshed, which appeared not to involve any signatories to the deal, came just over two weeks after the United Nations / African Union peacekeeping force UNAMID ended years of operations.

While former rebels have committed to laying down their arms, decades of conflict have left the vast, impoverished region awash with weapons and divided by bitter rivalries over land and water.

In South Darfur, fighting broke out early Monday between the powerful Arab Rizeigat tribe and the Fallata, a cattle and camel-herding people who trace their roots to the Fulani of western Africa.

"The clashes between the Rizeigat tribe and the Fallata tribe have killed 55 people, and wounded 37 others," local Fallata leader Mohamed Saleh told AFP.

Saleh said that several homes were torched in the attack, which appeared to be in revenge for Fallata tribe's killing of a Rizeigat member around a week ago.

- 'Deficient' peace deal -

The fighting in South Darfur comes after at least 83 people were killed in clashes between rival ethnic groups on Saturday and Sunday in El Geneina, the capital of West Darfur state.

On Monday, SUNA news agency quoted the West Darfur governor as saying the death toll was yet to be confirmed as uncounted bodies remained at the scene of the fighting.

Aid group Save the Children urged all sides to lay down their arms "before the situation gets out of control."

The charity's Sudan director Arshad Malik said the wounded had overwhelmed healthcare centres and that "we have seen disturbing photos of injured and dying people on the floors and corridors of hospitals".

The group warned that "many children will have been separated from their parents, and will now be at risk of exploitation".

Sudanese authorities said they had sent a "high-profile" delegation to calm tensions.

The vast north-east African country has been undergoing a fragile transition since the April 2019 ouster of president Omar al-Bashir following mass protests against his rule.

The Sudanese Professionals Association, an umbrella group which spearheaded the protests against Bashir, said Sunday that the violence showed the "deficiencies" of the peace deal, saying it failed to address the root causes of the conflict.

Bashir, who is in custody and on trial in Khartoum, is wanted by the International Criminal Court for alleged genocide and war crimes in Darfur.

- 'Ugly face of war' -

Darfur endured a bitter conflict that erupted in 2003, leaving roughly 300,000 people dead and 2.5 million displaced, according to the UN.

The war had erupted when ethnic minority rebels rose up against Bashir's Arab-dominated government, which responded by recruiting and arming a notorious Arab-dominated militia known as the Janjaweed.

The main conflict has subsided over the years but ethnic and tribal clashes still flare periodically, largely pitting semi-nomadic Arab pastoralists against settled farmers.

On December 31, the UNAMID force formally ended its 13 years of operations in the region. It plans a phased withdrawal of its approximately 8,000 armed and civilian personnel within six months.

Fearing deadly violence, Darfur residents held protests in late December against UNAMID's departure.

The UN secretary-general's spokesman Stephane Dujarric called on Sudanese authorities to "bring an end to the fighting".

"Escalating inter-communal violence has resulted in scores of deaths and injuries, the displacement of nearly 50,000 people and the destruction of property," he said in a statement.

Jan Egeland, secretary-general of the Norwegian Refugee Council, warned that "the ugly face of war" could return to Darfur and called for more aid and protection.

"International support must not fade," Egeland tweeted.

Sudan's transitional government, a power-sharing arrangement between generals and civilians, signed a peace deal with rebel groups in Sudan's main conflict zones, including Darfur, in October.

Gibril Ibrahim, leader of the Justice and Equality rebel movement, wrote on Twitter that the violence in West Darfur was "a human tragedy".

Another rebel leader, Mini Minawi, urged implementation of the peace deal called for "reconciliation" between Darfuri tribes.

Two groups have refused to join the peace deal, the latest in a string of agreements, including the Sudan Liberation Movement (SLM) faction led by Abdelwahid Nour, which is believed to have considerable support in Darfur.