Heavy fighting broke out between the Rwandan and Congolese military in troubled eastern DR Congo after an incursion by Rwandan troops, according to a Congolese army spokesman.
Investigations are underway to see if the clashes occurred because of a problem with border demarcation.
The fighting occurred on Tuesday "on Congolese soil" close to the Rwandan border at the foot of Mount Nikeno in North Kivu province, Major Guillaume Djike told AFP.
A senior officer in the DRC army, on condition of anonymity, told AFP: "There have been deaths on both sides."
Congolese army patrols exchanged fire with Rwandan army positions some 100 metres inside DR Congo before the two countries' capitals contacted each other and intervened, according to a military source.
The Congolese army initially thought it was fighting ex-M23 militia members, before realising it was the Rwandan army, said Djike.
The two neighbours have had a fractious relationship since the 1994 Rwandan genocide.
Some of those accused of involvement in the killings of an estimated 800,000 ethnic Tutsis and moderate Hutus in Rwanda set up militias in eastern DR Congo.
Rwanda has been accused of backing M23, a mainly ethnic Tutsi rebel group that emerged in this period.
The militia rebelled against the DR Congo in 2012, saying a peace accord signed in 2009 had not been respected by the government.
The militia was defeated in 2013, and hundreds of fighters fled the country.
An inquiry, led from Goma, the capital of the turbulent province of North Kivu, is expected to investigate whether Tuesday's violence was caused by confusion over the border line.
DR Congo's huge eastern region has long been wracked by violence, but fighting between government soldiers and militia groups, as well as inter-ethnic clashes, increased significantly in 2017.