Burundi's army says at least 87 people were killed Friday in the capital, Bujumbura, in what it describes as coordinated attacks on three military installations by unknown gunmen.
A military spokesman said Saturday that the dead included 79 "enemies," along with eight soldiers and policemen. He said 45 others had been captured in daylong firefights that extended into Saturday morning.
Horrified residents in several neighborhoods of the capital told Western reporters of emerging from hiding early Saturday to find dozens of corpses scattered in the streets.
Witnesses said some of the victims had been dragged from their homes by security forces and executed. Others described finding some of the dead with their hands bound behind their backs.
The army declined to address those allegations.
The fighting was seen as the worst since a failed coup attempt in May that came just weeks after two-term President Pierre Nkurunziza announced plans to seek a controversial third term. Analysts say nearly 250 people have been killed since then, and that some 200,000 residents have fled to nearby countries to escape the violence.
Critics, including opposition leaders, said a third Nkurunziza term would be unconstitutional. However, the incumbent and his allies insisted on the legality of the candidacy, arguing that Nkurunziza was chosen by lawmakers rather than voters to one of his two earlier terms and was therefore not violating the constitution's two-term limit.
Burundi's constitutional court later ruled in the president's favor, and Nkurunziza won a third five-year term in July, in polls that were boycotted by opposition groups.
The latest fighting drew sharp condemnation from the United States and the United Nations.
U.N. Secretary-General Ban Ki-moon urged leaders on both sides to "refrain from any further escalation of violence." He also warned that those responsible for human rights violations would be held accountable.
Samantha Power, the United States' U.N. ambassador, said "high-level political dialogue" needs to begin immediately between the government and the opposition "to defuse the situation" or else risk conditions "devolving into mass violence."
A U.S. State Department spokesman said Washington was "deeply alarmed by the attacks," adding that "we condemn this violence in the strongest possible terms, and we call on all sides to refrain immediately from violence."
The U.S. Embassy in Burundi on Friday ordered its personnel to shelter in place and advised Americans in Bujumbura to take similar precautions.
In a statement earlier this week, U.S. National Security Council spokesman Ned Price called for all sides in Burundi to reject violence and take part in an internationally mediated dialogue, outside the country, to resolve the crisis.
Price said the United States might impose sanctions on others who blocked peace efforts or resorted to violence.
Nike Ching at the State Department contributed to this report.
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