Rwandan soldiers killed four gunmen in a battle with a group of unidentified attackers who launched a cross-border raid from Burundi overnight, the army said Saturday.

Three Rwandan soldiers were injured in the fighting which erupted just after midnight in the southern district of Nyaruguru, just a short distance from the border, Lieutenant-colonel Innocent Munyengango told AFP.

The assailants fled back towards Burundi, leaving behind four dead among their ranks, as well as military equipment including weapons and communications equipment, he said.

"We reassure Rwandans that action will be taken against those responsible," he added.

Tension is high between the two neighbours, with Burundi accusing Rwanda of arming and training rebel groups to oppose late president Pierre Nkurunziza, who died suddenly earlier this month.

In November, Burundi vowed retaliation after accusing Rwanda of a surprise attack on the border that killed up to eight Burundian soldiers.

Rwanda has meanwhile accused Burundi of providing support for the Hutu rebel group FDLR opposed to the government of President Paul Kagame.

Munyengango did not directly accuse the Burundi army of taking part in the latest attack, but troops who spoke on condition of anonymity told AFP the gunmen were soldiers, and could have been as many as 50, given the intensity of the firefight.

A senior Burundi official said they were aware of the attack in Rwanda, and added in reference to the gunmen: "We deployed units at the border to intercept them.

"We have nothing to do with these groups that want to destabilise our neighbour," said.

He acknowledged that local commanders sometimes collaborated with rebel forces in the field, but stressed "that is not the army's policy".