GENEVA — “I could be your child; I beg you, save me,” the 16-year-old pleaded to police officers in Burundi. Seconds later, an officer put two bullets in his head.

The killing of the teenager, Jean Nepo, during an antigovernment protest on the streets of Bujumbura, the capital, was one of hundreds of summary killings cited by United Nations investigators in a report issued on Tuesday. The report documented serious abuses during the last 18 months that could amount to crimes against humanity.

“The responsibility for the vast majority of these violations should be laid at the door of the government,” the investigators wrote, urging international action to contain a crisis they said threatened the peace and security of the surrounding region in Central Africa.

The report documented 564 summary killings in Burundi from April 2015 to the end of August, but the report’s three authors wrote that “this is clearly a conservative estimate.” Their inquiry began in December and included two visits to Burundi.