Burundi has had soldiers in eastern Congo for years, initially to hunt down Burundian rebels there but, more recently, supporting Kinshasa's army in battles against M23.
Any showdown over Bukavu, some 200 km (125 miles) south of Goma, which they seized this week, could exacerbate ethnic tensions and openly pit national armies against each other, analysts told Reuters.
Although Rwanda denies accusation from the United Nations and others that it has sent troops in Congo, it says it will do anything necessary to defend itself. It accuses Congo's military of joining forces with Hutu-led militias it says are bent on slaughtering Tutsis in Congo and threatening Rwanda.
Burundi, whose ruling party draws heavily from the country's Hutu community, has had troops in Congo since 2021 and tensions have simmered between the two nations since Burundi accused Rwanda of masterminding a failed coup attempt in 2015.
"Today there are thousands of Burundian soldiers who are fighting against the M23 on the front lines. Some people say 8,000, others say 10,000," said Jason Stearns, a former U.N. investigator in Congo and currently a professor at Simon Fraser University.
M23 is the latest in a series of Tutsi-led rebellions that have emerged in Congo's east since the official end of a string of conflicts between 1996 and 2003 that sucked in most of Congo's neighbouring countries and killed 6 million people, mostly from hunger and disease.
Earlier this month, Burundian troops fought a fierce battle in Nungu, in North Kivu, losing dozens of men after they were overrun by M23, four sources told Reuters.
"The Burundians have (since) pulled back to South Kivu," Stearns said. "They are among the forces that are trying to block the advance of M23 and the Rwandan army north of Kavumu."
Kavumu, a town 35 km north of Bukavu, is home to the city’s airport and hosts a number of Congolese drones and other aircraft.
A U.N. source and Rwanda's government gave similar figures for Rwandan troops in Congo.
A Burundian official said the number of Burundian troops in Congo was "of this order" and said the military had received an increasing number of requests for help from Congo's military in recent years.
"Our country has also paid a heavy price, which is why we have asked our two neighbours to sign a ceasefire and negotiate," the official said, asking not to be named due to the sensitivity of the matter. Burundi's foreign minister did not respond to requests for comment. A spokesperson from the Congolese army did not respond to requests for comment.
Like Rwanda, Burundi has sought to heal deep divisions between its Tutsi and Hutu populations.
Under the peace deal that ended Burundi's 1993-2005 civil war, which killed 300,000 people, its military put in place an ethnic quota system that shared positions between Hutus and Tutsis equally.
Diplomats and experts said there is a risk that Burundian military units fighting in eastern Congo could fracture in fighting against the Tutsi-dominated and Rwandan-backed M-23.
Josaphat Musamba, PhD student at Ghent University, said Rwanda believes Burundi is harbouring Hutu rebels near their border.
"As soon as Rwanda thinks it is a threat, that becomes a threat to other countries," he said.
