BUJUMBURA, Burundi (AP) — The son of a human rights activist in Burundi was killed after being arrested Friday, a witness said, as international concern grew that a bloodbath might be imminent in the central African country.

Lily Mbonima said his brother Welly Fleury Nzitonda was killed because of their father's work. Nzitonda's death is part of an unremitting wave of killings in Burundi that has prompted international outcry and warnings that Burundi could be on edge of a violent upheaval.

A witness, who insisted on anonymity for fear of reprisals, said she was with Nzitonda when police arrested him. When police saw Nzintonda's identification card they assaulted him and ordered her to leave, she said. Nzitonda's body was found in an empty house in the Mutakura neighbourhood in a pool of blood.

Nzitonda's father, Pierre Claver Mbonimpa, who opposed Nkurunziza's bid for a third term in office, survived an assassination attempt earlier this year. Last month, Mbonimpa's son-in-law was killed in front of his home.

The U.N. human rights chief estimates that at least 198 people have been killed in Burundi since late April, when President Pierre Nkurunziza announced his bid that was ultimately successful for a third term in office. The bid was opposed locally and internationally. At least 13 people have died since last Saturday alone, with many coming from opposition strongholds. More than 200,000 people have fled Burundi.

The prosecutor of the International Criminal Court warned of a worsening security situation and said perpetrators would face justice.

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

Nkurunziza set a five-day deadline on Monday for civilians to turn in weapons or face tough action from the police.

Samantha Power, the U.S. ambassador to the United Nations, on Thursday quoted the president of the Burundian Senate, Révérien Ndikuriyo, as saying: "You tell those who want to execute the mission: on this issue, you have to pulverize, you have to exterminate - these people are only good for dying. I give you this order, go!"

She said the U.S. is concerned that Nkurunziza's ultimatum "will trigger widespread violence beginning this coming weekend."

In a statement, Bensouda warned that: "Any person in Burundi who incites or engages in acts of mass violence including by ordering, requesting, encouraging or contributing in any other manner to the commission of crimes falling within the jurisdiction of the International Criminal Court is liable to prosecution before this court."

France said in statement it's also very concerned about the continuing deterioration of the situation in Burundi and requested a U.N. Security Council on Monday to discuss the turmoil.

"We call upon all Burundians, government and opposition to show restraint and to engage in political dialogue, the only way to overcome the current crisis," the statement said.

The U.N. secretary-general condemned Friday's killing and said he is alarmed by the escalating violence in Burundi. Ban Ki-moon's spokesman said Ban also condemns public statements that appear to be aimed at inciting violence, calling them "reprehensible and dangerous."

Although the current violence appears to be political, Burundi has a history of deadly conflicts in which the country's Hutu and Tutsi ethnic groups targeted each other. Nkurunziza took power in 2005 near the end of a civil war in which some 300,000 people were killed between 1993 and 2006.