The Rapid Support Forces and allied militias launched an offensive on el-Fasher, the provincial capital of North Darfur province, on Sunday, the Resistance Committees in the city said. Dozens of other people were wounded in the attack, said the group, which tracks the war.
There was no immediate comment from the RSF.
El-Fasher, more than 800km southwest of the capital, Khartoum, is under the control of the military, which has fought the RSF since Sudan descended into civil war more than two years ago, killing more than than 24,000 people, according to the United Nations, though activists say the number is likely far higher.
The RSF has been attempting to seize el-Fasher for a year to complete its control of the entire Darfur region. Since then, it has launched many attacks on the city and two major famine-hit camps for displaced people on its outskirts.
The city is now estimated to be home to more than one million people, many of whom have been displaced by the ongoing war and previous bouts of violence in Darfur.
The RSF grew out of the notorious Janjaweed militias, mobilised two decades ago by then-president Omar al-Bashir against populations that identify as Central or East African in Darfur. The Janjaweed were accused of mass killings, rapes and other atrocities.
The attacks on el-Fasher have intensified in recent months as the RSF suffered battlefield setbacks in Khartoum and other urban areas in the county's east and centre.
Sunday's attack came less than a week after a two-day attack by the RSF and its allied militias on the city and the Zamzam and Abu Shouk camps killed more than 400 people, according to the United Nations.
Last week's attack forced up to 400,000 people to flee the Zamzam camp, Sudan's largest, which has become inaccessible to aid workers, said UN spokesman Stephane Dujarric.
