Pierre Chanel Ntarabagnyi said that former rebel members had died during clashes between security forces and armed groups.

The head of the rights group, Pierre Claver Mbonimpa, accused that police agents or intelligence services summarily killed at least 22 militants of the National Liberation Forces (FNL).

"Small groups travel around and loot, kill and rape, and when police face these groups, there are exchanges of fire and sometimes police are killed or their people," the spokesman said.

"When these small groups invade a small village, they kill systematically... The police must use force to pursue them and it is normal that there are deaths," he added.

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

"What's happening in Burundi is very alarming," Pierre Claver Mbonimpa, president of the Association for the Protection of Detainees and Human Rights said.

"Many people are arrested by the police, and then they disappear and are found dead some time afterwards. In September alone we counted 22 victims of extra-judicial executions."

Several hundred members of other parties have been arrested since May while around 120 were being held for alleged offences against state security, according to rights group in Burundi.

The United Nations is worryied 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.

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.

At least 30 people were killed in the recent violence. Burundi has only just came out from a 13-year civil war that left more than 300,000 dead