Three soldiers and eight rebels were killed in overnight fighting as members of Burundi's National Liberation Forces (FNL) attacked a military base.
Pierre Nkurunziza has promised new peace talks
Army spokesman Maj Adolphe Manirakiza told AFP that the incident took place in the Western Bubanza province.
"We think the FNL are seeking attention now that we are having presidential elections," Maj Manirakiza said.
Parliament will choose a new president on Friday, the last step in a peace process to end 12 years of civil war.
Vying for attention
Pierre Nkurunziza, whose party won recent parliamentary elections, is the only candidate and is expected to be formally endorsed as president by the two houses of parliament.
His former Hutu rebel group, the Forces for the Defence of Democracy, dominates the chamber following polls in June and July.
If confirmed, Mr Nkurunziza will be Burundi's first democratically elected leader since 1993 and will be inaugurated next week.
"A group of FNL rebels yesterday (Wednesday) night attacked a military post which provides security to the Kagwema displacement site, we lost three soldiers and killed five rebels," Maj Manirakiza told AFP.
On Wednesday rebels also staged a mortar attack on the capital city Bujumbura, the army said. However, the bombs fell harmlessly in a field.
"There are indications that the FNL is planning to intensify attacks from now until the inauguration, just to show that it is still present on the ground and maybe put some pressure on the new government to talk to them," Maj Manirakiza said.
The FNL is Burundi's last active rebel group and Mr Nkurunziza has vowed to engage them in peace talks.
"The first priority is to engage in talks with the FNL and to conclude hopefully a ceasefire agreement with that movement," Mr Nkurunziza said in a speech to parliament.
Some 300,000 people were killed in the country's the civil war between Hutu rebels and the Tutsi-dominated army.