The United States on Wednesday called on all armed forces in Sudan to immediately cease attacks in al-Fashir, North Darfur, as Sudan’s army battles the paramilitary Rapid Support Forces (RSF) for control of the vast north African country.

In a statement, the U.S. State Department said it was “alarmed by indications of an imminent offensive” against the city by the RSF and its militias that would endanger civilians, including hundreds of thousands of displaced people.

It also said it was troubled by RSF’s reported razing of multiple villages west of al-Fashir as well as “indiscriminate” aerial bombing and humanitarian aid limits by Sudanese Armed Forces.

“The leaders of the SAF and RSF and their affiliated militias face a choice – escalate the violence and perpetuate the suffering of their people while risking the disintegration of their country, or cease attacks, allow unhindered humanitarian access, and prepare in good faith for negotiations to end this war and restore power to the people of Sudan,” department spokesman Matthew Miller wrote.

Attacks around al-Fashir have shattered a truce that protected it from a year-old war, leading to warnings of a new wave of inter-communal violence and humanitarian risks for 1.6 million residents crammed into the North Darfur capital.

Al-Fashir is the last major city in the vast, western Darfur region not under control of the paramilitary Rapid Support Forces (RSF). 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.

The fight for al-Fashir, a historic center of power, could be more protracted, inflame ethnic tensions that surfaced in the early-2000s conflict in the region and reach across Sudan’s border with Chad, say residents, aid agencies and analysts.