Gunmen executed nine people in Burundi's capital hours before police launched house-to-house searches for weapons on Sunday, amid international fears of fresh bloodletting in the central African nation.

Hundreds of police and soldiers ringed the opposition flashpoint Mutakura district of the capital Bujumbura early Sunday to start a widely feared crackdown on "enemies of the nation."

After months of violence, troubled Burundi risks sliding into civil war, a leading think tank has warned, issuing a "conflict alert" after a string of battles between gunmen.

"Burundi again faces the possibility of mass atrocities and civil war," the Brussels-based International Crisis Group (ICG) warned in a report issued late Thursday.

Uganda's defense minister is in Burundi in the latest bid to kickstart peace talks between rival factions in the impoverished central African country, presidential officials said Thursday.

Burundi has been rocked by violence linked to President Pierre Nkurunziza's successful but contested bid for a third term in office.

At least seven gunmen have been killed in battles with security forces in Burundi, a governor said Wednesday, the latest in a string of clashes in the troubled central African nation.

Gunmen and soldiers fought Tuesday in the eastern district of Gitega, a government stronghold, the province's governor Venant Manirambona said.

Gunmen and security forces battled in a series of deadly clashes in Burundi, police said Tuesday, the latest violence since the controversial re-election of President Pierre Nkurunziza.

At least seven people were killed and 15 others wounded in clashes since Monday, the latest in months of unrest triggered by Nkurunziza's successful bid to win a third term in office.

The U.N. human rights chief warned Monday that Burundi risked sliding back into civil war after a dramatic rise in killings, arrests and detentions in the troubled central African country.

"Almost every day, dead bodies are found lying on the streets of some of Bujumbura's neighborhoods," Zeid Ra'ad Al Hussein said in a statement.

The head of Burundi's armed forces on Friday survived an assassination attempt in the capital Bujumbura, police said, adding that at least seven other people were killed in the attack.

"There was an attack against the armed forces chief, General Prime Niyongabo, but happily he was unharmed," Burundi's deputy police chief, General Godefroid Bizimana, told Agence France Presse.

At least four people have died in renewed clashes in Burundi, where tensions remain high following President Pierre Nkurunziza's controversial re-election, officials and witnesses said Wednesday.

Police and witnesses said clashes erupted overnight in several parts of the central African nation's capital Bujumbura, with the night marked by sporadic gunfire and explosions. Police blamed armed "insurgents" for the violence.

Burundi's president on Wednesday called for groups that threaten national security to be "destroyed", setting a combative and hardline tone as he begins a controversial third term in office.

In a speech read out on state media, Pierre Nkurunziza said young people would be given "patriotic, theoretical and practical training" to work alongside the central African nation's security forces.

Burundi security forces used iron bars and acid to force confessions and crush opposition during President Pierre Nkurunziza's successful bid for a third term in office, Amnesty International said Monday.

Testimonies recorded by Amnesty in the report accuse both the police and National Intelligence Service (SNR) of carrying out "torture and other ill-treatment" since April 2015 against people suspected of participating in protests against Nkurunziza's controversial re-election bid.