At least 40 people have been killed and 58 injured in three days of clashes in Sudan's West Darfur state, the United Nations said Monday.
After clashes at the weekend, residents of El Geneina, capital of West Darfur and close to the border with Chad, said gunfire erupted in the city at dawn on Monday, with shells exploding in the suburbs.
Plumes of smoke rose high into the sky as people fled the violence, witnesses reported.
"Since April 3, 40 people have been killed," the UN humanitarian coordination office OCHA said in a statement.
It said the clashes pitted Arab groups against the non-Arab Massalit ethnic community in the city of El Geneina, where the situation remained "tense".
Sudan's top security body, the defence council, late Monday declared a state of emergency in West Darfur state and deployed troops to the region.
"On Monday, we woke up to the sound of gunfire," Abdelrahman Ahmed, an eyewitness said. Clashes "spread to the western suburbs".
It is unclear what sparked the latest violence.
Women and children fleeing
The vast Darfur region was ravaged by a civil war that erupted in 2003, leaving around 300,000 people dead and 2.5 million displaced, according to the UN.
It flared when ethnic minority rebels rose up against dictator Omar al-Bashir's Arab-dominated government.
Khartoum responded by unleashing a notorious Arab-dominated militia known as the Janjaweed from among the region's nomadic tribes.
The conflict has subsided over the years, and the latest in a string of peace deals was agreed in October.
But after years of conflict, the region is awash with automatic weapons. Clashes still erupt, often over land and access to water.
Residents long displaced during the worst years of the war are returning to find others have occupied their lands.
Eyewitnesses said fighting still raged on Monday afternoon, as thick smoke billowed over El Geneina.
"We stayed in our homes, but we've been hearing gunfire close by -- a shell landed at our neighbour's home," said Adam Issa, another resident.
The West Darfur Doctors' Committee said an ambulance carrying wounded victims was attacked.
Clashes often pit nomadic Arab pastoralists against settled farmers from non-Arab ethnic groups.
Saleh Issa, another El Geneina resident, said he saw a "cloud of smoke" covering swathes of the city.
"Some residents from these neighbourhoods have fled towards our area -- most of are women and children," he added.
Sudan is in the midst of a rocky transitional period following the toppling of long-time president Omar al-Bashir in April 2019 off the back of mass protests against his rule.
The transitional government has pushed to build peace with rebel groups in Sudan's main conflict zones, including Darfur.
On December 31, the UN and African Union ended a 13-year peacekeeping mission in Darfur, even as residents feared further violence.
More than 200 people were killed in clashes in January, in some of the worst bloodshed the region had witnessed in years.