A Congolese army officer and four Ugandan rebels were sentenced to death on Monday for the killing of a colonel credited with making great strides in restoring peace to the east of the DR Congo.

The military tribunal in North Kivu province concluded that soldiers plotted to kill Mamadou Ndala and that Ugandan rebels carried out the killing, its chief judge Colonel Joseph Maya Mokako told AFP.

Ndala commanded wide respect for leading operations against the M23 rebel movement, which was finally defeated in November last year.

Ndala, who embodied hopes for a disciplined, effective and republican army in the DR Congo, was killed in an ambush in January.

At the time the army was preparing a major operation against another group, the Allied Democratic Forces, which had operated in the east since fleeing Uganda in 1995 and were blamed for massacring some 120 civilians in October and November 2013.

Aged just 35, Ndala had won over the local population in North Kivu, reconciling them with an army generally known for its ineffectiveness and indiscipline, as well as abuses against civilians.

He was promoted to general after his death.

The DR Congo is observing a moratorium on the death penalty, so the sentences will be commuted to life imprisonment.

Lieutenant-Colonel Birotso Nzanzu was sentenced to death and expelled from the army, Colonel Maya Mokako told AFP by telephone from the North Kivu capital Goma.

Four ADF members including their leader Jamil Mukulu got the death sentence in absentia.

Another officer, Lieutenant-Colonel Joker Kamuleta, was sentenced to 20 years behind bars and a civilian got 15 years.

The court also meted out sentences of between one and five years to a dozen other defendants while acquitting five including Lieutenant-Colonel Tito Bizuru and his deputy Captain Moise Moussa Banza, a former aide to Ndala accused of stealing property belonging to him.

Multiple armed groups still operate in the mineral-rich region, which has been in turmoil for the best part of the past two decades.