A witness says a son of a prominent human rights activist was killed after being arrested, part of an unremitting wave of killings in Burundi that the prosecutor of the International Criminal Court warned of a worsening security situation.

"It is with grave concern that I note the increasing risk of violence in Burundi," Fatou Bensouda said.

Lily Mbonima said Friday his brother Welly Fleury Nzitonda was killed because of their father's work.

A witness who insisted on anonymity for fear of reprisals says she was with Nzitonda when police arrested him. The witness says when police saw Nzintonda's identification card assaulted him and ordered her to leave. Nzitonda's father, Pierre Claver Mbonima, who opposed Nkurunziza's bid for a third term in office, survived an assassination attempt earlier this year.