KINSHASA (Reuters) - Militia fighters decapitated about 40 police officers after an ambush in central Democratic Republic of Congo, local officials said on Saturday, the deadliest attack on security forces since an insurgency erupted in the region last year.

The Kamuina Nsapu militants struck on Friday as the police convoy drove from the city of Tshikapa in Kasai province to Kananga, the capital of Kasai-Central province, said Francois Madila Kalamba, speaker of the Kasai provincial assembly.

“They were apprehended by the militia members and they decapitated about 40,” Kalamba told Reuters. He added that witnesses said the fighters spared the lives of six police officers because they spoke the local Tshiluba language.

The militia fighters, who are often armed with machetes but rarely carry firearms, made off with arms and vehicles during the raid, Kalamba added.

Corneil Mbombo, president of the Civil Society of Kasai, a provincial activist group, also said about 40 officers had been decapitated following the ambush. The provincial governor and national police spokesman could not be reached for comment.

The insurgency, which has spread to five provinces, poses the most serious threat yet to the rule of President Joseph Kabila, whose failure to step down at the end of his constitutional mandate in December was followed by a wave of killings and lawlessness across the vast central African nation.

Friday’s attack follows government reports of a wave of surrenders by fighters in neighboring Kasai-Central province in recent days. The Interior Ministry said on Saturday that 400 fighters had surrendered this week in the province.

But as the insurgency has spread, the fighters operating under the name Kamuina Nsapu appear to operate increasingly independently and without a clear leadership structure. Some recent violence appears to be ethnic score-settling.

More than 400 people have been killed in the violence, according to the United Nations, and the government said on Tuesday that 67 police officers and many soldiers had died in the clashes.

Many of the dead have been dumped in mass graves. The United Nations said this week that it had identified 10 alleged mass grave sites and was investigating seven others.

The military’s top prosecutor announced last week that seven soldiers had been charged in connection with a video that appears to show soldiers massacring suspected militia members, including for murder and mutilation.

Two U.N. officials, one U.S. citizen and the other of Swedish nationality, and four Congolese accompanying them were also kidnapped last week by unknown assailants in Kasai-Central. They have yet to be located.