Security forces in the Democratic Republic of Congo have killed at least 36 Burundian refugees during clashes over plans to send some of them home.
Maman Sidikou, the head of MONUSCO, the UN's peacekeeping mission in the country, said in a statement on Saturday that at least 36 refugees had been reported killed and more than 100 injured.
He called for a swift investigation and urged Congo's security forces to use force as a last resort only.
Josue Boji, a Democratic Republic of Congo interior ministry official, said troops had tried to disperse the refugees by "firing in the air, but were overwhelmed" when the group responded by throwing stones during Friday's confrontation.
Police and soldiers opened fire as the refugees protested over the resettlement plan and tried to free some of their arrested compatriots in the town of Kamanyola in eastern Congo, sources told the Reuters news agency.
Activist Wendo Joel said the refugees had seized a weapon and killed a soldier, though that account was not confirmed by other sources.
A Burundian refugee told AFP news agency: "I saw people falling down, men, women and children who were completely unarmed."
Al Jazeera was unable to independently verify the death toll.
A local army spokesman, Dieudonne Kasereka, told Reuters there had been clashes between soldiers and refugees armed with knives and machetes, but that he did not know if there were any deaths.
'Shocked and saddened'
The UN refugee agency (UNHCR) also called for an investigation of "this tragic incident," saying in a statement that it was "in shock and saddened".
UNHCR spokesman Andreas Kirchhof told Al Jazeera that the "circumstances are not clear". He added that teams have been sent to the area, including medical staff to treat those injured.
Burundian Foreign Minister, Alain-Aime Nyamitwe, on Twitter described the incident as a "shooting" and said "explanations are needed".
Jason Stearns, of the Congo Research Group, told Al Jazeera that while it is still very difficult to know what happened, the context is important.
"The Congo has not only harboured many refugees, but many people that the Burundian government has deemed to be its opponents," Stearns said.
"The Burundian government, along with the Congolese government have, at least in the past, targeted those opposition members."
More than 400,000 refugees have fled Congo's neighbour Burundi since violence erupted there in April 2015 when President Pierre Nkurunziza said he would seek a third term in office, a move his opponents said was unconstitutional.
Earlier this month, the UN released a report accusing Burundi's government of crimes against humanity, including executions and torture, and urged the International Criminal Court (ICC) to open a case "as soon as possible".
Burundi's government firmly rejected the allegations, accusing the UN investigators of being "mercenaries" in a Western plot to "enslave African states".
Source: Al Jazeera and news agencies