The head of the UN mission in the Democratic Republic of Congo has said he is sorry his forces reacted too slowly to stop the murder of at least 30 civilians in the east of the country in June.
Armed men attacked the village of Mutarule in South Kivu province, in an ethnic dispute between June 6 and 7.
Human Rights Watch said in a report on Thursday that the villagers had begged UN troops to intervene as the attack unfolded.
Martin Kobler, the head of the organisation's mission in the DRC known as MONUSCO, said: "It is a very difficult situation and I express my regrets for the inactivity of MONUSCO. It really is regrettable."
"We have taken measure with our forces and our mobile operational base to have night patrols, to respond quickly to the needs of the population ... to be able to save the lives of citizens in similar incidents," he added.
The mission had launched an investigation into its response to the incident, military spokesman Felix Basse told the Reuters news agency.
"We have deployed a human rights investigation team in the area and established an operational base at Mutarule to prevent further attacks," Basse said .
The Congolese army and UN troops needed to determine what went wrong and make sure such atrocities do not happen again on their watch, said Anneke Van Woudenberg, an Africa advocacy director at HRW.
Attacks on civilians are common in Congo's lawless eastern provinces. Ethnic disputes and competition over access to land and Congo's vast mineral wealth fuel the violence.
Congolese authorities have also launched an investigation into the massacre in South Kivu and have arrested two army officers for reacting too slowly. Government spokesman Lambert Mende rejected HRW's criticism of the army.
"Why are HRW talking about the entire Congolese army? There were officers at fault and we have made the necessary arrests. They will be judged," Mende told Reuters.
The UN's DRC mission is the largest in the world, with 22,000 uniformed personnel at a cost of $1.5bn a year.