Fighters from the Kamwina Nsapu group attacked a police convoy.
Six policemen who spoke the local Tshiluba were freed, but the rest were killed, Kasai Assembly President Francois Kalamba said.
The unrest in Kasai began last August, when security forces killed the Kamwina Nsapu leader.
Friday's attack targeted a police convoy travelling between Tshikapa and Kananga.
The state Governor Alexis Nkande Myopompa said an investigation had been launched into the killings.
The UN says 400 people have been killed and 200,000 displaced in the Kasai region since Jean-Pierre Pandi, the Kamwina Nsapu leader, was killed.
This came two months after Kamwina Nsapu launched a bid, in June 2016, for him to be officially recognised as a local chief and for state bodies to withdraw from the region.
Why have 10 mass graves been found in Kasai?
The UN says it has identified 10 mass graves where those killed in the unrest have been buried, as well as another seven suspected mass burial sites.
Two UN experts, an American and a Swede, were kidnapped in the area two weeks ago with four Congolese colleagues and are still missing.
DR Congo is in a state of increasing political uncertainty as President Joseph Kabila remains in power beyond the end of his mandate ,which expired last December.
Elections are now due to be held before the end of this year, but no firm date has been set.

Ofgem announced it had granted an electricity interconnector licence to Scandinavian consortium NorthConnect.
The developers plan to build a Â£1.3bn power cable between Boddam in Aberdeenshire and Eidfjord in Norway.
The project aims to link hydro power from Norway with wind energy from Scotland.
It is scheduled to start operating from 2022.
NorthConnect have said that the cable will have a capacity of 1.4GW - about 25% of Scottish peak demand.
Having been awarded the licence, NorthConnect still has to undertake further regulatory steps.
It can apply for Ofgem's "cap and floor" regime, which regulates how much money a developer can earn once in operation.
Alternatively, the consortium can ask for an exemption from certain European rules that apply to interconnectors.
Last year, Aberdeenshire Council approved an application by the consortium to build an electricity converter station and underground cables at Boddam.
The converter station and onshore cables are required to connect the interconnector cable to the National Grid.
Interconnectors are transmission cables that allow electricity to flow from one country to another.
Ofgem has said they can lower consumer bills by accessing cheaper power and boosting UK energy supplies.
Responding to the Ofgem announcement, WWF Scotland director Lang Banks said: "If the UK and the rest of Europe are to move to a 100% renewable future then greater use of interconnectors is a sensible way forward.
"Sharing different renewable resources between nations would help drive down climate emissions much faster than relying on domestic action alone.
"However, it shouldn't be an excuse for any country to halt the development of their own renewable capacity.
"A European-wide 'supergrid' would also bring the double benefits of security of supply and a reduced need to build lots of expensive new nuclear or fossil fuel power stations."
NorthConnect is jointly owned by Swedish utility Vattenfall and Norwegian companies E-CO Energi, Agder Energi and Lyse.
In 2013, Perth-based energy giant SSE withdrew from the project, saying it wanted to focus instead on its markets in Britain and Ireland.
Militia fighters in DR Congo have decapitated about 40 police officers in an ambush in the central province of Kasai, local officials say.
[NEXT_CONCEPT]
UK regulators have given the green light to plans for a 400-mile subsea power cable linking Scotland and Norway.