Paris prosecutors have opened a preliminary investigation into allegations that French troops beat up, or stood by while others beat up two people in the Central African Republic, a judicial source said Monday.

French authorities launched an enquiry last week into "voluntary group violence" as well as sequestration and failing to help persons in danger over the attack.

Five soldiers are suspected of assaulting two locals -- or looking on while the men were assaulted -- at a military outpost in the working-class PK12 district of CAR's capital Bangui in early 2014.

The defence ministry announced earlier this month that the five troops had been suspended from the Sangaris force deployed in the poor former French colony to quell sectarian violence.

The ministry said it had also launched disciplinary action against the soldiers, which could lead to their eviction from the military.

Four other soldiers are also subject to sanctions because "they were aware of the facts but said nothing", the ministry added.

The investigations will seek to understand why the soldiers targeted the locals and why no one stopped the assault, which was revealed by France's Ouest-France daily.

A senior officer reported the affair to his superiors in late April.

"The preliminary investigation will focus on searching for and identifying the victims," a judicial source told AFP.

The Sangaris force is already the subject of three investigations into separate allegations of sexual abuse of children in Central Africa.

UN peacekeepers also face sex abuse allegations.

France launched the Sangaris military operation in in December 2013 after an outbreak of inter-communal violence between Muslim and Christian militias that killed thousands.

The mission is due to end in December after a progressive draw-down.