The CMA said in late July it was holding dozens of prisoners in Kidal, a city it controls, after dismantling a base of the GATIA militia.
Fighting between the ex-rebels of the Coordination of Movements of Azawad (CMA) and the pro-government Platform group has worsened in recent weeks in northern Mali, in repeated violation of a 2015 ceasefire.
The CMA said in late July it was holding dozens of prisoners in Kidal, a city it controls, after dismantling a base of the GATIA militia -- part of Platform -- and killing several of its fighters.
But Guillaume Ngefa, director of the human rights division of the UN mission to Mali, said Thursday that eight of those being held were children and possibly child soldiers.
And he detailed what he said were human rights abuses perpetuated by both sides.
"We were able to establish that the armed groups had minors in their ranks," Ngefa said in a statement.
"What is extremely concerning is that among the 33 people currently held prisoner by the CMA in Kidal, eight are children."
United Nations investigators had carried out seven different missions to the north and discovered evidence of mass graves and rampant rights abuses, he added.
These included "summary executions, kidnappings and torture, destruction of property and theft," he said, elaborating on Saturday's statement of the discovery of the mass graves.
The number of bodies and the cause of death of those buried remains unknown.
Ngefa also raised the possibility of compensation for the victims of the abuses. The UN would support families wishing to lodge legal cases against the perpetrators, while continuing to investigate 33 outstanding complaints, he said.
The US State Department said on July 31 it was "greatly disturbed at reports of reprisal killings of civilians and the discovery of unmarked grave sites..."
If the allegations proved founded, it added, "the perpetrators must be brought to justice".
The conflict dates back to 2012, when key cities in northern Mali fell under the control of jihadist groups linked to Al-Qaeda, who exploited an ethnic Tuareg-led rebel uprising.
The armed groups accused of abuses by the UN operate in areas where Mali's army is absent or has a very limited presence.
Although the Islamists were largely ousted by a French-led military operation, attacks have continued on UN and French forces, civilians and the Malian army.
Allegations of collusion between the former rebels and jihadists continue to circulate.