Witnesses told them that in a single incident in North Kivu province, the rebels killed 17 people they suspected had informed the Congolese army about M23 positions.

Others were said to have been killed in subsequent attacks in the area.

DR Congo has accused neighbouring Rwanda of supporting the M23 - which Rwanda denies.

Three weeks ago, the country's two leaders agreed on a "de-escalation process" in eastern Congo.

The M23 leapt to prominence in 2012, before being driven out in a joint UN-Congolese offensive. It resumed fighting last November.