The US government urged its citizens on Sunday to immediately leave Burundi after 87 people were killed in the capital Bujumbura, many reportedly shot dead execution-style.
The State Department said it has ordered the departure of non-emergency personnel and dependents of US government employees from Burundi because of escalating violence linked to President Pierre Nkurunziza's disputed third term.
Violence was ignited on Friday after attacks by assailants on three army installations, Burundian officials said on Saturday.
Army spokesman Colonel Gaspard Baratuza said eight security officers were among those killed during and after Friday's assault.
"The final toll of the attacks yesterday is 79 enemies killed, 45 captured and 97 weapons seized, and on our side eight soldiers and policemen were killed and 21 wounded," Baratuza was quoted as saying by the AFP news agency.
A climate of fear has engulfed Bujumbura, though the streets were reportedly calm on Sunday.
"So far the violence has been political," reported Al Jazeera's Mohammed Adow, adding it has sparked "fears that civil war in Burundi could be rekindled".
About 300,000 people were killed between 1993 and 2006 during the civil war.
"There is a police operation that is continuing in the suburbs of the capital," Adow said, explaining that young men have been the primary target.
An eyewitness told the Associated Press he counted 21 bodies with bullet wounds to the head in the Nyakabiga neighbourhood on Saturday morning.
Some of the dead had their hands tied behind their backs.
Q&A: 'We continue to believe in Burundi'
Police spokesman Pierre Nkurikiye said there were "no collateral victims" during Friday's clashes.
"All the deaths were attackers killed in the joint sweep operation of the army and police," Nkurikiye said. "The enemy was neutralised."
The violence is linked to President Nkurunziza's third term in office, which many Burundians and foreign observers had opposed as unconstitutional and in violation of a peace accord that ended the civil war.
More than 300 people have now been killed since April and about 215,000 others have fled to neighbouring countries, according to the United Nations.
Several hundred people have also been imprisoned for opposing Nkurunziza's re-election in July this year.
Source: Al Jazeera and agencies