UN chief Antonio Guterres said he was "outraged" after two people were killed and several others injured when UN peacekeepers opened fire during an incident in the eastern Democratic Republic of Congo on the Uganda border on Sunday.
The UN force MONUSCO, admitted that some of its peacekeepers had opened fire "for unexplained reasons", adding that arrests had already been made.
Guterres was "saddened and dismayed" to learn of the shooting, a UN statement said.
"The Secretary-General stresses in the strongest terms the need to establish accountability for these events.
"He welcomes the decision of his special representative in the Democratic Republic of the Congo to detain the MONUSCO personnel involved in the incident and to immediately open an investigation," it added.
Video of the incident, shared on social media showed men, at least one in police uniform and another in army uniform, advancing towards the immobilised UN convoy behind a closed barrier in Kasindi.
The town is in eastern DR Congo's Beni territory on the border with Uganda.
After a verbal exchange, the peacekeepers appeared to open fire before opening the barrier and driving through while people scattered or hid.
"During this incident, soldiers from the intervention brigade of the MONUSCO force returning from leave opened fire at the border post for unexplained reasons and forced their way through," the UN mission in Kasindi said in a statement earlier on Sunday.
"This serious incident caused loss of life and serious injuries."
The Democratic Republic of Congo "strongly condemns and deplores this unfortunate incident in which two compatriots died and 15 others were injured according to a provisional roll," government spokesman Patrick Muyaya said in a statement.
- 'Shocked and dismayed' -
The government said it launched an investigation with MONUSCO to establish who was responsible, why the shooting took place and would ensure "severe penalties" are given.
The UN envoy in the Democratic Republic of Congo, Bintou Keita, said she was "deeply shocked and dismayed by this serious incident", according to the mission's statement.
"In the face of this unspeakable and irresponsible behaviour, the perpetrators of the shooting have been identified and arrested pending the conclusions of the investigation which has already begun in collaboration with the Congolese authorities," MONUSCO said.
The UN mission said the troops' home countries had been contacted so legal action could be commenced promptly, with the involvement of witnesses and survivors, which could lead to exemplary penalties.
- Deadly region -
Earlier Barthelemy Kambale Siva, the North Kivu governor's representative in Kasindi, said that "eight people, including two policemen who were working at the barrier, were seriously injured" in the incident.
Kambale Siva, interviewed by AFP, did not say why the UN convoy had been prevented from crossing.
There are more than 120 militias operating in the DRC's troubled east. The UN first deployed an observer mission to the region in 1999.
In 2010, it became the peacekeeping mission MONUSCO -- the United Nations Organization Stabilization Mission in the Democratic Republic of the Congo -- with a mandate to conduct offensive operations.
There have been 230 fatalities among the force, according to the UN.
- Calls for UN forces to leave -
Last week, deadly demonstrations demanding the departure of the United Nations took place in several towns in eastern DRC.
A total of 19 people, including three peacekeepers, were killed.
Anger has been fuelled by perceptions that MONUSCO is failing to do enough to stop attacks by the armed groups.
UN under-secretary-general for peace operations Jean-Pierre Lacroix was in the central African country on Saturday to "talk to the Congolese authorities", he said.
"(They would) examine ways in which we can both avoid a recurrence of these tragic incidents and, above all, work better together to achieve our objectives," he said.
"We hope that the conditions will be met, in particular the return of state authority, so that MONUSCO can complete its mission as soon as possible. And to leave room for other forms of international support."