Burundi Army force acting on a tip-off ambushed a group of armed men in the western Rugazi region, sparking an hour-long fire fight that forced the gang to retreat to a nearby forest, said Gaspard Baratuza, the army spokesman.

The civilian was mistakenly fired by soldiers who laid an ambush at night. The spokesman said they thought were bandits.

"Unfortunately, they were civilians who were taking a pregnant woman to hospital, one of them, a student, were killed by mistake," he said.

Another group of armed men walked into a police patrol and two of them were arrested, a local administrator said.

The region where the incidents had happened is a stronghold of the National Liberation Forces (FNL), a former rebel group.

The National Liberation Forces had joined government last year, but its leaders Agathon Rwasa fled after local ballot in May which the opposition accused Burundi authorities of rigging.

Burundi has recently been challenged by a series of killings which government referred to two opposition groups whose leaders have gone into hiding of being behind the attacks.

The United Nations is worried about the situation. "The situation is extremely worrying. The risk of a return to violence should not be underestimated", said Charles Petrie, the UN secretary general's outgoing special representative to the country.