Clashes near El-Fasher, a relative haven in Darfur largely spared from the last 12 months of war in Sudan, have raised fears of a further expansion of the devastating conflict.
Since fighting erupted last 15 April between Sudan's army and paramilitary Rapid Support Forces, the already war-ravaged western region of Darfur has been the site of horrific violence, including reports of mass ethnic-based killings.
But North Darfur's El-Fasher, the last state capital not under RSF control in the vast Darfur region, had been a site of comparative stability and a key humanitarian refuge.
"Clashes have taken hold of the countryside west of the city," a local human rights defender told AFP by phone, requesting anonymity for fear of reprisal.
"Air strikes have pounded (the area) and everyone is terrified," she added.
The local resistance committee, one of many pro-democracy groups organising aid across the country, said on Saturday an RSF-allied militia had set fire to six villages west of El-Fasher.
At least 10 civilians were killed in the attacks, according to another activist committee.
United Nations Secretary-General Antonio Guterres was "deeply concerned" by reports indicating "an attack on El-Fasher may be imminent", his spokesperson said on Saturday.
In a statement, he said "such an attack would be devastating for civilians in the city," which has functioned as "a humanitarian hub for the UN that ensures lifesaving assistance" across Darfur.
The immense region, the size of France, is home to a quarter of Sudan's 48 million people and -- like much of the country -- is on the brink of famine.
The US ambassador to the UN, Linda Thomas-Greenfield said in a post on X on Saturday that she was "gravely concerned by reports of escalating conflict" in El-Fasher that risked "humanitarian catastrophe in an already dire situation".
As violence rages through the countryside, residents in North Darfur have reported an influx of displaced people moving towards the city, in the hope that it might prove safer than their villages.
But even within the city on Saturday, "we could hear the sound of clashes," local activist Adam told AFP, requesting to be identified only by his first name.
The UN said last year that 80,000 people had already sought shelter in El-Fasher's "overcrowded school buildings, or were left out in the open".
Peace in the key city was similarly threatened last year, before local armed groups successfully negotiated a makeshift truce -- acting as peacekeepers in what the UN called "a fragile status quo".
According to a UN expert report, the RSF had "refrained from attacking after informal negotiations with the Darfurian armed movements there".
The groups had also coordinated desperately needed aid from eastern Sudan, before supply lines broke down, experts told AFP.
Intermittent attacks had already escalated in recent weeks, with eyewitnesses reporting increased clashes between the army and the RSF.
But unrest has soared since the two most powerful armed groups -- led by Darfur governor Mini Minawi and Sudan's finance minister Gibril Ibrahim -- pledged to fight alongside the army.
Both men are key allies of army chief Abdel Fattah al-Burhan.
On Thursday, they released a statement saying the RSF, commanded by Burhan's former deputy Mohamed Hamdan Daglo, had "taken advantage of the forces' position of neutrality and defence".
"There can no longer be neutrality," they declared, vowing to "fight along with our allies, the patriots and the armed forces against the RSF militias and their hired helpers".
Both sides have been accused of war crimes, including targeting civilians, indiscriminate shelling of residential areas and torture.
Earlier this month, military jets conducted strikes that "failed to avoid civilian locations and killed dozens of civilians", said US envoy for Sudan Tom Perriello.
Across the country, the year-old war has killed many thousands, including 10,000 to 15,000 people in a single West Darfur town, UN experts found.
It has also pushed the country of 48 million people to the brink of famine, devastated already fragile infrastructure and driven more than 8.5 million people from their homes.