The United Nations has sounded the alarm about a possible imminent attack on al-Fashir, in Sudan’s North Darfur, as the global body seeks to reduce tensions in the last major city in the region not under the control of the Rapid Support Forces (RSF).

A spokesperson for UN Secretary-General Antonio Guterres said on Friday that the RSF was reportedly encircling al-Fashir, “suggesting a coordinated move to attack the city may be imminent”.

“Simultaneously, the Sudanese Armed Forces appear to be positioning themselves,” the spokesperson said in a statement.

Guterres called on all parties to refrain from fighting in the al-Fashir area, said the spokesperson, adding that his envoy on Sudan, Ramtane Lamamra, was working to de-escalate the tensions.

“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 spokesperson added.

War erupted in Sudan one year ago between the Sudanese army and the paramilitary RSF, creating the world’s largest displacement crisis.

The RSF and its allies swept through four other Darfur state capitals last year and were blamed for a campaign of ethnically driven killings against non-Arab groups and other abuses in West Darfur.

Residents, aid agencies and analysts have warned that the fight for al-Fashir, a historic centre of power, could be more protracted.

It could also further inflame ethnic tensions that surfaced in the early-2000s conflict in the region and reach across Sudan’s border with Chad.

The United States on Wednesday also called on all armed forces in Sudan to immediately cease attacks in al-Fashir.

Meanwhile, earlier on Friday, the UN’s High Commissioner for Human Rights Volker Turk expressed “grave concern” over fighting near the city. A statement from Turk’s office said dozens of people have been killed in and around al-Fashir in the past two weeks.

“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 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.”

Top UN officials warned the Security Council last week that some 800,000 people in al-Fashir were in “extreme and immediate danger” as worsening violence threatens to “unleash bloody intercommunal strife throughout Darfur”.

The UN has said nearly 25 million people — half of Sudan’s population — need aid and some eight million have fled their homes.

Donors last week pledged more than $2bn for the war-torn country at a conference in Paris.