United Nations officials expressed "grave concern" Friday over "increasingly alarming reports" about the escalating fighting in Sudan's North Darfur region, echoing warnings by US diplomats of an imminent attack.

A "coordinated attack" on the city of El Fasher "may be imminent," Secretary-General Antonio Guterres' spokesperson's office warned.

"An attack on the city would have devastating consequences for the civilian population. This escalation of tensions is in an area already on the brink of famine," the office said in a statement, reiterating previous calls from Guterres to avoid fighting there.

At least 43 people, including women and children, have been killed in fighting between the Sudanese Armed Forces (SAF) and the Rapid Support Forces (RSF) since April 14, when the RSF began its advance towards El Fasher, it said.

Earlier on Friday, the UN's High Commissioner for Human Rights Volker Turk also expressed his "grave concern" over fighting near the city .

Turk "is gravely concerned by the escalating violence in and around El Fasher city, North Darfur, where dozens of people have been killed in the past two weeks," a statement from his office said.

Millions have been displaced in the country since fighting began last year between the SAF forces of General Abdel Fattah al-Burhan and FSR paramilitaries under General Mohamed Hamdan Daglo.

Until recently, El Fasher -- the last Darfur state capital not under RSF control -- had been relatively unaffected by the fighting, hosting a large number of refugees.

But since mid-April, bombardments and clashes have been reported in the city and surrounding villages.

"Civilians are trapped in the city, the only one in Darfur still in the hands of the SAF, afraid of being killed should they attempt to flee," the rights chief's office's statement said.

"This dire situation is compounded by a severe shortage of essential supplies as deliveries of commercial goods and humanitarian aid have been heavily constrained by the fighting, and delivery trucks are unable to freely transit through RSF-controlled territory," it added.

Turk called for an immediate de-escalation and an end to the conflict, as well as an "investigation into all alleged violations and abuses of international human rights law and international humanitarian law," the statement said.

The statement from Guterres' office said his personal envoy, Ramtane Lamamra, "is engaging with the Parties to de-escalate tensions in El Fasher."

The UN and United States have warned the breakdown of the fragile peace in El Fasher would be catastrophic.

The city functions as the main humanitarian hub in the vast western region of Darfur, home to around a quarter of Sudan's 48 million people.

On Wednesday, US diplomats had warned of a possible "imminent" paramilitary offensive on El Fasher.