They blamed the paramilitary Rapid Support Forces (RSF) for the attack on Al-Luweib village on Monday, when mourners were gathered in a tent. The RSF has not yet commented on the incident.
According to reports, many people died before reaching the hospital in Al-Obeid, a strategic city that connects the capital Khartoum to the western Darfur region.
Fighting has intensified in the oil-rich Kordofan region, and nearly 20,000 people fled to Al-Obeid last week after the RSF seized the town of Bara, 30 km (18 miles) north of the city.
That town fell around the same time as Al-Fasher, the army's last stronghold in Darfur.
Since then, there have been reports of widespread killings, sexual violence, abductions, and mass looting by RSF fighters in Al-Fasher.
The UN said it had also received reports of summary executions of civilians by RSF fighters in Bara.