UN peacekeepers in Democratic Republic of Congo will begin using unarmed drones on a trial basis to monitor its war-torn east, the head of peacekeeping operations told Reuters on Sunday.

UN peacekeeping chief Herve Ladsous, in Paris to attend France's Independence Day celebrations, told Reuters a deal signed on Friday with an unnamed company would allow for a "complete picture of what is happening" on the ground.

Thick forests, rugged terrain and few roads have complicated peacekeepers' efforts to control the area.

"We have just signed a commercial contract for the UAVs, and I say UAVs, not drones, as they are unarmed," Ladsous said, referring to unmanned aerial vehicles.

"This is a major innovation. For the first time the UN is going into state-of-the-art, 21st-century technology."

Drone flights are planned to begin in September.

UN peacekeeping troops have been in eastern Congo for more than a decade, and the MONUSCO force is currently 17,000 strong - the largest UN force in the world.

But the complex conflict has dragged on, killing millions through violence, famine and disease since the 1990s. That has led the U.N. to create a new "intervention brigade" - part of the MONUSCO force but charged with the task of not merely peacekeeping but taking proactive steps against rebel groups.

It has already begun patrolling and is approaching full strength, Ladsous said.

Most peacekeepers from Tanzania and South Africa are already in place, and those from Malawi are expected to be deployed at the end of July or early August to complete the 3,000-strong force, he said.

Ladsous defended the brigade's mission to take a more active approach to neutralising rebel groups. UN peacekeeping principles stipulate impartiality and "non-use of force except in self-defence and defence of the mandate."

"Neutrality, impartiality: that is the case for classic peacekeeping," said Ladsous.

"How can you be neutral or impartial to those terrible armed groups who have been for years now, a decade or more, killing civilians, raping women, recruiting child soldiers? No, you cannot be neutral."

Congo has been afflicted by an insurgency by M23 rebels in its border area with Rwanda and Uganda in the last year, and UN experts accused Rwanda of sending troops and weapons across the border to support the M23. Rwanda denies the accusation.