UN Secretary General António Guterres says the attack on UN peacekeepers in the Democratic Republic of Congo may be a war crime, according to a UN spokesperson.
"He underscores that any attack directed against United Nations peacekeepers may constitute a war crime and calls upon the Congolese authorities to investigate these incidents and swiftly bring those responsible to justice," deputy UN spokesperson Farhan Haq said in a statement.
Three United Nations peacekeepers and at least 12 demonstrators have been killed in escalating anti-UN protests in eastern DR Congo, officials said on Tuesday.
Anger has been fuelled by perceptions that MONUSCO, the UN mission in the Democratic Republic of Congo, is failing to do enough to stop attacks by armed groups.
Crowds on Monday stormed a MONUSCO headquarters and supply base in Goma, the chief city in North Kivu province, and the protests spread on Tuesday to Beni and Butembo to the north.
Butembo police chief Colonel Paul Ngoma said three peacekeepers there -- two Indians and a Moroccan -- had been killed and another injured, while seven demonstrators had died and several others were wounded.
Earlier, government spokesman Patrick Muyaya said on Twitter that "at least five people (were) dead, about 50 wounded" in Goma.
The security forces had fired "warning shots" at protesters to stop attacks on UN personnel, he said.
MONUSCO is one of the world's biggest peacekeeping operations.
But it has regularly come under criticism in the troubled east, where many accuse it of failing to do enough to end decades-long bloodshed.
More than 120 armed groups roam the volatile region, where civilian massacres are common and conflict has displaced millions of people.
In Monday's unrest, hundreds of people in Goma blocked roads and chanted hostile slogans before storming MONUSCO's headquarters and a supply base there.
Protesters smashed windows and looted valuables, while helicopters airlifted UN staff from the premises and security forces fired teargas in a bid to push them back.
The unrest in Goma continued on Tuesday, with the fatal shooting of a man near the supplies base, an AFP correspondent saw.
Congolese security forces held back a crowd outside the facility, some of whom carried placards such as "Bye-bye, MONUSCO."
At Goma's CBCA Ndosho Hospital, the head of administration, Serge Kilumbiro, told AFP that 28 people had been admitted with gunshot wounds on Monday and eight more on Tuesday.
In Beni, about 350 kilometres (215 miles) to the north, soldiers deployed on the road leading to the MONUSCO base there on Tuesday, while protesters burned tyres. Shops, markets and petrol stations were closed.
In the town of Butembo, security forces dispersed protesters who had gathered in front of a MONUSCO base, witnesses said.
Ngoma, the local police chief, said some youths were "armed".
Resurgence of rebel militia
The latest protests come after the president of the senate, Modeste Bahati, told supporters in Goma on July 15 that MONUSCO should "pack its bags".
They coincide with the resurgence of the M23 -- a militia that lay mostly dormant for years before resuming fighting last November.
The rebels have since made significant advances in eastern Congo, including capturing the North Kivu town of Bunagana on the Ugandan border.
The UN first deployed an observer mission to eastern Congo 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.
It has a current strength of about 16,300 uniformed personnel, according to the UN.
MONUSCO on Monday said it "vigorously condemned the attack" on its premises in Goma, "carried out by a group of looters on the sidelines of a demonstration which, in addition, was banned by the mayor of Goma".
In a statement on Monday, Khassim Diagne, the deputy special representative of the UN secretary general to MONUSCO, said the peacekeepers were there to protect civilians.
"The incidents in Goma are not only unacceptable but totally counterproductive," he said.
On Tuesday, Guterres also said the United Nations would work with Congolese authorities to investigate the deaths of demonstrators.
(FRANCE 24 with AFP)
