"The Burundi defense forces contingent officially entered DRC (today)... under the forces pooling framework put forward by the heads of state of the EAC," East African Community, said Lieutenant Marc Elongo, army spokesman in South Kivu province.
The contingent, comprising "a large number of soldiers," is under the command of the DRC forces and currently stationed at a training facility in the Uvira area, he told AFP.
The Burundians and their DRC counterparts "are tasked with hunting down all foreign and local armed groups in order to restore peace" in eastern Congo, he said.
South Kivu's head of military operations, General Ramazani Fundi, urged the public "to be calm and work honestly with loyalist forces in order to put an end to this activity by irregular forces," said Elongo.
In June, EAC leaders decided to set up a regional force that would work with the DRC's army to quell armed groups roaming the east of the country.
An estimated 120 armed groups are active in the mineral-rich region, many of them a legacy of wars that flared in the final years of the 20th century.
The idea of a regional force has run into criticism within DR Congo.
Opponents point to past chapters of meddling in the east by neighbors, and instead are clamoring for reforms and funding for the national armed forces.
Critics include 2018 Nobel Peace co-laureate Denis Mukwege, a gynecologist and surgeon who has treated thousands of women victims of rape in the conflict region.
The EAC comprises Burundi, the DRC, Kenya, Rwanda, South Sudan, Tanzania and Uganda.