Dozens of people, including women and children, have been killed in Democratic Republic of Congo's South Kivu province.
The state's governor blamed the attack on a dispute over cattle.
"It is Congolese (Republic of the Congo) who have carried out these attacks. It was about a dispute over cows," South Kivu governor Marcellin Cishambo told the Reuters news agency. "The problem is that everyone in this area carries a weapon."
South Kivu in the DRC is home to members of a tribe who fled Burundi after the end of the 2005 civil war.
However, some Congolese Bafuliru tribe locals blame rebels from Burundi's National Liberation Forces (FNL) for the attack.
Accusation of military training
Burundi's military on Saturday denied its troops were operating in the war-torn east of DRC after Kinshasa-based press freedom group Journaliste en Danger (JED) accused them of going after two journalists in the area.
JED said two of its reporters had been investigating whether members of Burundi's ruling party's youth wing were undergoing military training in DRC's Kivu province.
"We categorically deny this report," Burundi's spokesperson Colonel Gaspard Baratuza told news agency AFP. "If any Congolese journalists have been harassed by soldiers, they are not Burundian soldiers."
Burundian soldiers, he added, were only on their side of the border.
Allegations that the ruling CNDD-FDD party was training its youth sector first came to light in April when an internal United Nations report was leaked.
President Pierre Nkurunziza vehemently denied the allegations, ordering the top UN official out of the country.
After decades of civil war, Burundi is still healing, but tensions are growing in the lead up to elections next year. Nkurunziza is expected to seek a third term in office despite a constitutional two-term limit.