The probe was established by the United Nations Human Rights Council last year to investigate abuses in Kasai, which was plunged into violence in September 2016after government troops killed a local chieftain, Kamwina Nsapu.
The chieftain was opposed to the Kinshasa government and now rebels fighting in his name are battling government forces and a pro-government militia called the Bana Mura.
"Some of the abuses committed by the defence and security forces, the Bana Mura militia and the Kamwina Nsapu militia constitute crimes against humanity (and) war crimes," the investigators said.
The report will be presented to the rights council next month.
It accused both government troops and militia members of targeting civilians in a "systematic or widespread manner," highlighting atrocities that include murder, mutilation, rape, sexual "enslavement" and other "inhuman acts".
"A very serious problem of impunity persists, given the scale and gravity of these crimes," added the investigators, who were granted access to Kasai to conduct the probe.
The unrest in Kasai has claimed more than 3,000 lives and displaced some 1.4 million people from their homes
, according to the Roman Catholic church, although this estimate is months old and an accurate picture of the conflict is notoriously hard to come by.
The crisis is the latest in what has proven to be one of Africa's most troubled nations, which has been devastated by two regional wars and countless rebel insurgencies since the mid-1990s.
The new report came as the country's bishops on Tuesday called for a "rapid and massive intervention" to save children facing starvation during the dry season in Kasai.
Archbishop Marcel Madila warned that "if the children are not helped now, we risk thousands of deaths," in a statement issued by Catholic charity Caritas.
The charity cited statistics by the UN's children's agency Unicef showing that at least 770,000 children under five
in the region suffer from acute malnutrition,400000 of whom are facing the threat of death.
