Heavy fighting between a rebel group and Congolese army in eastern Democratic Republic of the Congo (DR Congo) over the weekend caused civilian casualties and raised tension, a UN spokesperson said Monday.

The M23 rebel group used heavy weapons and fired indiscriminately into densely populated areas, said Farhan Haq, associate spokesperson for UN Secretary-General Ban Ki-moon.

An M23 mortar shell hit the Ndosho section of the region's main city of Goma on Saturday, killing a civilian and injuring five others, he said.

A UN peacekeeper was also injured when the M23 fired shells towards the mission's position on the Munigi Heights, said Haq, noting that the UN Mission in the Democratic Republic of the Congo (MONUSCO) fired back to suppress M23 positions.

The death of the civilian by M23 fire in Goma sparked violent demonstrations throughout the town. Demonstrators attacked a UN mission transit camp near Goma airport. Two people were killed and six injured when the Congolese authorities and MONUSCO tried to contain the crowd who sought forceful entry into the camp.

Haq said the UN peacekeeping mission and Congo's national army launched a joint investigation into the two demonstrators' death.

"I condemn all attacks causing deaths and injuries among the civilian population, and remind all parties to the conflict that the indiscriminate or deliberate attack against civilians is a war crime," said the UN Humanitarian Coordinator in DR Congo, Moustapha Soumare, over the weekend.

Asked about actions taken by the UN mission, Haq said it was focused on ensuring that civilians in Goma be protected and out of the line of fire from armed groups.

The past year witnessed repeatedly clashes between the M23, along with other armed groups, and government forces in eastern DR Congo. The group briefly occupied Goma in November 2012. More than 150,000 people have been displaced towards Goma since 2012, UN officials said.