UNMIS said the report indicated that on Sunday and Monday, the armed group travelling on camels, horses and in vehicles killed the 18 victims, wounded 16 others, and attacked and burned the villages of Dar es Salam, Jamali, Funfo, Tabeldyad, Um Djantara and Um Putrum in the Gereida area of South Darfur.
The UN mission said that the African Union (AU) had been notified of those reports and intended to investigate them, the UN News Service reported.
In another development, Jan Pronk, the UN Secretary-General's special representative in Sudan, said he had informed the Sudanese government that a UN expert panel monitoring the arms embargo was mishandled last weekend.
Pronk reported that he had sent a diplomatic protest to the Sudanese ministry of foreign affairs over the incident on Saturday, in which two members of the panel had been manhandled, forcibly restrained and suffered abrasions, despite the fact that they had clearly identified themselves and their capacity.
He said that on Thursday he met Sudan's state minister of foreign affairs, who said the behaviour of Sudanese military intelligence "had been wrong" and promised that the panel's mission would not be hindered any further.
Sudan's western region of Darfur has been ravaged by a conflict that erupted in February 2003 when rebels took up arms to fight what they said was discrimination and oppression of the area by the Sudanese government.
The government stands accused of unleashing militia - known as the Janjawid - on civilians in an attempt to quash the rebellion. Some 3.3 million people continue to be affected by the conflict, according to the UN, of whom 1.8 million are internally displaced and 200,000 have fled to neighbouring Chad.