UN Secretary-General Ban Ki-moon on Thursday called for a rigorous investigation after nine civilians including an IOM staff member were shot in Burundi, reportedly at close range.
Ban condemned the killings in which two police officers also died during exchanges of heavy gunfire in Bujumbura on Wednesday.
Residents told AFP that a Burundian journalist, Christophe Nkezabahizi, his wife and two teenage children were killed in their home.
Evariste Mbonihankuye, a staff member of the International Organization for Migration, was killed in the violence, a UN statement said.
The UN chief "urges Burundian authorities to undertake a rigorous and prompt investigation into the circumstances and motives behind these despicable crimes in order to ensure that their perpetrators are brought to justice," said a statement.
The violence followed the re-election of President Pierre Nkurunziza, whose campaign for a third term was denounced by the opposition as unconstitutional.
Last month, UN rights chief Zeid Ra'ad Al Hussein warned that Burundi risked sliding back into civil war and raised alarm over the dramatic rise in killings.
"Almost every day, dead bodies are found lying on the streets of some of Bujumbura's neighborhoods," he said.
"In many cases, the victims appear to have been killed by a bullet fired at close range. The bodies sometimes show signs of torture and are typically found with their hands tied behind their backs."
The UN Security Council in August demanded that Burundi's government immediately reopen talks with the opposition to pull the country away from the brink.
The 15-member council backed a plan by Ban to send a high-level UN official to Burundi, but the secretary-general has yet to appoint an envoy in the face of strong resistance from Nkurunziza.