Sudanese security forces took part in the killing of some 100 people in the war-torn Darfur region, the UN says.
The Border Intelligence Guards are said to be run by army intelligence
UN Human Rights Commissioner Louise Arbour says Border Intelligence Guards took part in eight raids this year during clashes between Arab groups.
"Attackers fired indiscriminately from the outskirts of the settlements with heavy machine-guns and rocket-propelled grenades," her latest report said.
Sudan has always denied reports that it has armed Arab militias in Darfur.
At least 200,000 people have died and 2m have been made homeless during the four-year conflict.
Black African rebels took up arms in 2003, saying they were being marginalised by the Arab-dominated government.
The Border Intelligence Guards, often recruited from the local population, are "known to be under the control of Military Intelligence", said the UN report.
The unit has previously been accused of working with the Janjaweed militia, responsible for widespread atrocities in Darfur, such as mass killings, rape and looting.
Between January and March this year, it sided with the Rizeigat Abbala group against its rivals, the Tarjum, in a land dispute, the report said.
The UN says there is no evidence that Sudan's government ordered the attacks but it says neither did Khartoum make any real effort to protect its civilians from the violence or to investigate those responsible.
Sudanese military identity cards were later found at the scene.
Ms Arbour has called for an independent investigation into the killings.
But the BBC's Imogen Foulkes says this is something she has asked for many times before in relation to the violence in Darfur and so far Khartoum has been unwilling to cooperate with independent human rights experts.