Six people including a police officer were killed in a clash Wednesday between police and members of a separatist religious sect in the southwest of the Democratic Republic of Congo, police said.

The three-hour clash occurred in the Atlantic port city of Boma, leaving four civilians dead as well as one policeman and a member of the Bundu Dia Kongo (BDM) cult, Boma police chief Colonel Zebedee Kipanga told AFP.

He said the policeman was lynched.

The police were carrying out a mission to stop BDM members from chasing foreigners from the area, Kipanga said.

The BDM, or "Kingdom of the Kongo" movement, seeks to restore a 15th-century Congolese kingdom inside pre-colonial boundaries comprising parts of modern-day DR Congo, the Republic of Congo, Angola and Gabon.

The fresh unrest came two days after clashes left four dead in the town of Kisantu about 130 kilometres (80 miles) from the capital Kinshasa on the road to Boma, an essential import-export axis in the sprawling central African country.

Ne Muanda Nsemi, the group's spiritual leader, was accused by the government of staging deadly attacks on state institutions between the end of 2016 and early 2017.

On March 30, several dozen of his supporters defied a ban on the assembly of more than 20 people imposed by authorities because of the COVID-19 pandemic, disrupting traffic on a busy street in Kinshasa.

Congolese police dispersed them with tear gas.