At least 100 people have been killed after a cross-border attack against the central African nation of Burundi from the Democratic Republic of Congo (DRC), AFP news agency reports quoting a military source.

A general in the Burundian army, speaking on condition of anonymity on Sunday, said the raid by the unidentified group had been thwarted after five days of heavy fighting in the border area north of the capital Bujumbura.

"After five days of non-stop military operations, the armed group which attacked Burundi has been wiped out by our security forces. In total, we killed 105 of them and captured four, out of a total of the 121 who entered Cibitoke province from the DRC," the general said.

"We also seized a 60mm mortar, five rocket launchers, machine guns and more than 100 assault rifles."

The general said the Burundian army had lost two soldiers. Other military sources said that around 12 soldiers were killed in the fighting.

There was no further confirmation of the claim, with Colonel Gaspard Baratuza, army spokesperson, declining to comment officially on the fighting "while military operations in the area are still in progress".

A previous toll given on Wednesday put the toll at 35 dead, including 34 rebels and one government soldier.

Burundian officials and witnesses said the group of unidentified fighters crossed into the country overnight on Monday from DRC's eastern Kivu region, a chronically unstable and resource-rich area that is home to dozens of rebel groups.

Baratuza said troops had seized documents allowing the army to get information on the size of the force and the identity of their leaders, but he did not disclose the identity of the group.