Rebel fighters have killed at least 40 policemen in the central Democratic Republic of Congo, in one of the deadliest attacks on security forces since a violent uprising began late last year.

The Kamwina Nsapu militia reportedly killed the police officers on Saturday and only spared those who spoke their local language.

"They ambushed the policemen as they travelled from Tshikapa to Kananga," Ambrose Muwasa, a senior security officer, told the Anadolu Agency.

"After capturing them, they started killing them and only spared six who spoke Tshiluba language."

Corneil Mbombo, the president of the Civil Society of Kasai, a province-wide activist group, told the Reuters news agency that the 40 officers had been decapitated.

The militia then reportedly fled with vehicles and guns belonging to the police.

The rebel group has been fighting DRC forces since August 2016, when security forces killed their leader - Kamwina Nsapu. The violence has since spread to five provinces, posing the biggest threat yet to the rule of President Joseph Kabila.

According to the United Nations, more than 400 people have been killed and rights groups have warned the military against excessive use of force.

Several mass graves, suspected to contain bodies of militia fighters, have been discovered by local and international human rights groups.

And last month, the rebel fighters attacked police only to have the army respond with fierce force, killing more than 100 of them in an act condemned by human rights groups.