A court in Burundi jailed four ex-generals to life in prison Friday for a failed coup, with nine others jailed for 30 years for their role in the unrest.

The small central African country descended into bloodshed in April when President Pierre Nkurunziza announced his intention to run for a controversial third term, sparking the failed coup in May.

The U.N. human rights chief voiced alarm Friday at the spiraling violence in Burundi over "extremely disturbing" allegations of security forces gang raping women, ethnic killings and mass graves.

Burundi has been sliding deeper into violence since President Pierre Nkurunziza announced in April his intention to run in elections last July.

Former Burundi defense minister Cyrille Ndayirukiye and four other defendants have pleaded guilty to involvement in a failed coup last May during a trial of 28 police and military figures, judicial sources said Tuesday.

Ndayirukiye, number two among the coup plotters, and two other army generals and two police commissioners cited the violent repression of anti-government protests to justify their attempted takeover of power.

Burundian President Pierre Nkurunziza said Wednesday he would fight proposed African Union peacekeepers if they set foot on Burundian soil, defying intense global pressure to accept the force.

"Everybody should respect the borders of Burundi. If the troops are in violation of this decision, they will have attacked Burundi, and each Burundian must stand up to fight them," Nkurunziza said in a speech broadcast on state radio.

Burundi faces mounting regional pressure to accept African Union peacekeepers it calls an "invasion force", diplomats said Wednesday, with the issue a key point of talks due in Uganda next week.

The 54-member African Union said last week it would send a 5,000-strong force to halt violence that has sparked fears Burundi is sliding back towards civil war, and has pledged to send troops despite Burundi's fierce opposition.

Burundi's parliament is debating Monday a proposed African Union peacekeeping mission that Bujumbura has rejected as an "invasion force".

Lawmakers are expected to oppose the planned force, which was proposed by the African Union last week as international alarm grows over spiraling violence in the tiny central African nation.

The African Union will send a 5,000 strong force to stop violence in troubled Burundi, giving Bujumbura a four-day deadline to agree but warning it would send troops anyway.

The move comes as international alarm grows over spiraling violence in the tiny, landlocked nation which has been mired in bloodshed since April, sparking concern it was sliding towards civil war.

The U.S. Treasury Department on Friday sanctioned four Burundi nationals for allegedly "fueling violence" in the strife-torn African country.

Burundi descended into bloodshed in April, when President Pierre Nkurunziza announced his intention to run for a controversial third term, which he went on to win in July.

The African Union has agreed to send troops to stop violence in Burundi, a senior official said Friday, after talks in which the 54-member bloc warned it would not allow "another genocide".

The move comes as international alarm grows over spiraling violence in the tiny, landlocked nation which has been mired in bloodshed since April, sparking concern it was sliding towards civil war.

Burundi is at risk of imminent "atrocities" that could lead to a Rwanda-style genocide failing urgent international action, the United Nations human rights chief said Thursday.

Saying Burundi was "on the cusp of a full-blown civil war", Zeid Ra'ad Al Hussein urged the situation be brought before the International Criminal Court.