African Union rights investigators have expressed "great concern" over an "escalation of violence" in Burundi after witnessing some of the heaviest fighting in the troubled country for months.

A team from the AU-backed African Commission on Human and Peoples' Rights was in Burundi from December 7-13 on a fact-finding mission.

U.S.-based advocacy group Refugees International said that men and boys in Rwanda's Mahama camp, run by the United Nations and Rwandan authorities, were being recruited into "non-state armed groups" and faced threats if they refused.

The charity added that the Burundian recruits are trained in Rwanda and efforts are made to send them back to Burundi via neighboring Democratic Republic of Congo.

The streets of Burundi's capital Bujumbura were calm as life returned to normal Sunday, two days after nearly 90 people were killed in some of the worst violence in months of unrest.

The army said 87 people were killed -- 79 "enemies" and eight soldiers, according to Colonel Gaspard Baratuza -- during and after coordinated assaults on three military installations early on Friday morning.

U.S. rights group Human Rights Watch called Sunday for a "serious and independent" inquiry to be carried out urgently into the latest violence in Burundi that left at least 87 people dead.

"This is by far the most serious incident, with the highest number of victims, since the start of the crisis in April," Carina Tertsakian, HRW's researcher for Burundi, said in a statement.

Heavily-armed gunmen launched coordinated assaults on two army barracks in the Burundi capital on Friday leaving dozens dead, mostly attackers, in the worst unrest since a failed May coup, a senior military officer said.

The fighting began at around 4:00 am (0600 GMT) when "heavily armed men" attacked a base at Ngagara in the north of Bujumbura as well as a military training college in the south, according to the officer, who spoke to AFP on condition of anonymity.

U.N. Secretary General Ban Ki-moon said Monday that deploying peacekeepers to Burundi was an option to quell the violence but recommended that a U.N. team be first sent to help bolster dialogue.

Ban laid out three options to address the crisis in a letter to the U.N. Security Council, which has adopted a resolution calling for measures to prevent mass atrocities in Burundi.

Burundi is at risk of genocide, warns Thomas Perriello, U.S. Special Envoy for the Great Lakes Region of Africa, days after President Barack Obama slapped sanctions on the public security minister and three others.

Burundi descended into violence in April after President Pierre Nkurunziza launched a bid for a third consecutive term in office, despite concerns over the legality of such a move.

An African regional bloc has pulled its headquarters out of crisis-hit Burundi after months of violence to ensure the "safety and security of its staff," it said in a statement.

"The situation in Burundi is seriously worrying," the 12-nation International Conference on the Great Lakes Region (ICGLR) said in a statement seen Friday.

At least six people were killed and several others wounded in the latest violence in Burundi's capital, police and witnesses said Monday, a week after the launch of a crackdown search for weapons.

"There have been several armed criminal attacks in many neighborhoods of Bujumbura which were apparently coordinated," a senior police officer said, confirming that six people were killed in separate attacks overnight Sunday.

Rwandan President Paul Kagame has accused Burundi's leaders of carrying out "massacres" on their people in his most critical speech yet of the crisis in the troubled neighboring state.

"People die every day, corpses litter the streets... How can the leaders allow their population to be massacred from morning to night?" Kagame said, speaking in Kinyarwanda on Friday, in a speech heard by AFP on Sunday.