More than 3,300 people have been killed and 1.4 million forced to flee their homes in Kasai since the start of the insurrection by the Kamuina Nsapu militia, which wants the withdrawal of military forces from the area.
A spokesman for the U.N. peacekeeping mission in Congo told reporters in the capital Kinshasa that the graves were identified across six different sites during a joint mission to western Kasai last week with Congolese military investigators.
The government has blamed the militia for mass graves discovered in neighbouring Kasai-Central province and also claims that some of the alleged mass graves identified by U.N. investigators have not turned out to contain bodies.
But witnesses in Kasai-Central interviewed in March by Reuters said they had seen army trucks dumping bodies and the United Nations has repeatedly accused Congolese troops of using excessive force.
The government denies that its troops have systematically used excessive force, although a court convicted seven soldiers last week for murdering suspected militia members in a massacre that was caught on video.
Reporting By Aaron Ross; Editing by Tim Cocks from Reuters