"We call of the Congolese authorities to shed light as quickly as possible on the unacceptable actions and identify those responsible, who should answer for the actions," French Foreign Affairs Ministry spokesperson Romain Nadal said in a statement.
The video, which has been posted on social media, appears to show DRC soldiers in uniform fire on civilians, including three women, and finish several of them off while they lie on the ground, leaving about 20 corpses.
France "condemns the murderous violence" in Kasai, the statement says.
On Sunday the US State Department called on the DRC government to launch an inquiry into the incident.
"The weight of proof lies with the accusers," DRC government spokesperson Lambert Mende told the AFP news agency.
On Saturday the DRC government claimed the film was a "montage" but later accepted that some of its troops may have committed "excesses" in fighting the rebellion in Kamwina Nsapu, in the south of Kasai Central province.