Al-Fashir is the last major city in the vast, western Darfur region not under control of the paramilitary Rapid Support Forces, RSF.
RSF and its allies swept through four other Darfur state capitals last year. The paramilitary group was blamed for a campaign of ethnically driven killings against non-Arab groups and other abuses in West Darfur.
Residents, aid agencies and analysts say the fight for Al-Fashir, a historic center of power, could be more protracted and inflame ethnic tensions that surfaced in the early-2000s.
Al-Fashir's population includes an estimated half a million people displaced during Sudan’s earlier conflict, when the army, assisted by Arab militias that evolved into the RSF, put down a rebellion by non-Arab rebel groups.
Approximately 500,000 more people moved into the city during the fighting that broke out between the army, known as the Sudanese Armed Forces, SAF, and the RSF in Sudan’s capital Khartoum in April 2023. This happened after long-simmering tensions over integrating the rival forces came to a head.
As the conflict spread to other parts of the country, local leaders brokered a truce in Al-Fashir, with the RSF confined to eastern areas of the city while the former rebel groups stayed neutral. But the arrangement fell apart after the paramilitary group took the town of Melit this month, effectively blockading al-Fashir.
Witnesses say SAF has reinforced supplies and troops, including through an air drop to its base in the city, unlike in other state capitals where soldiers quickly fled.
Two prominent former rebel groups, Minni Minawi's Sudan Liberation Army (SLA) and Jibril Ibrahim's Justice and Equality Movement, said they would also defend against the RSF.
Many non-Arabs in Al-Fashir are gripped with fear.
Mohamed Gasim, a 39-year-old resident of Al-Fashir said locals "don’t know what to do" because there is danger in the city "but leaving is more dangerous."
Prior to the collapse of the truce in Al-Fashir, there were occasional skirmishes that killed more than 220 people in the city over the last 12 months, according to Ismail Khareef, an activist in Abu Shouk, one of the displacement camps that dot the city.
Clashes on April 16 left at least 18 dead, Khareef said.
Gunfire and projectiles, including from army warplanes, have fallen on homes, the activist and other residents say.
Since the start of the month, at least 11 villages on Al-Fashir's outskirts have been razed, according to satellite imagery obtained by the Yale Humanitarian Research Lab.
The United Nations estimates that at least 36,000 people have been displaced.
Local activists and an SLA spokesperson blamed the RSF and allied militias, who have been known to use arson in past attacks, including in West Darfur. They said that survivors reported around 10 people killed and that the attackers used ethnic insults.
The RSF denied attacking Al-Fashir and said it was careful to keep clashes away from civilians, accusing SAF and allied groups of attacking the city on the outskirts. The paramilitary group has previously denied responsibility for ethnic violence in Darfur.
SAF did not immediately respond to requests for comment.
Awadalla Hamid, the director of Practical action in Darfur, says Al-Fashir has not had functioning running water or power lines for a year, adding that only one public hospital is functioning, while displaced people are crammed into schools and public buildings.
Jerome Tubiana, an expert on Darfur and advisor to medical charity MSF, said all-out fighting "risks already complicating further humanitarian access, at a time where available data shows al-Fashir is suffering of an extremely serious food crisis."
Since the fighting between SAF and RSF began, only small quantities of aid have entered Al-Fashir, the only army-approved conduit for shipments to other parts of Darfur.
Residents say that though markets are functioning, the RSF's control of the main road has caused prices for fuel, water and other goods to soar.
Recent tensions and violence around Al-Fashir have also raised concerns about a wider spillover.
The former rebel groups fighting alongside SAF hail from the Zaghawa tribe, which reaches across the border into Chad, counting Chadian leader Mahamat Idriss Deby as a member.
Arab and non-Arab tribes like the Zaghawa have long clashed over land and valuable resources in Darfur, according to analysts.
Complicating matters is the entrance of the forces belonging to Musa Hilal, a leading Arab commander from the early 2000s and rival of RSF commander Mohamed Hamdan Dagalo, known as Hemedti, despite hailing from the same tribe.
A spokesperson confirmed a video of Hilal addressing forces in North Darfur on Monday but said that it was too soon to say if the forces would join the fight in Al-Fashir or elsewhere.
Jonas Horner, an independent Sudanese analyst said "even if there was a cease-fire between SAF and RSF this is way beyond them. There are scores being settled and tensions being renewed."