A Swede and an American working for the United Nations in the Democratic Republic of the Congo's restive Kasaï region were investigating allegations of soldiers murdering civilians when they were kidnapped five days ago, according to reports.

The experts – Michael Sharp, a US citizen, and Zaida Catalan, of Swedish nationality – had been instructed by the UN Security Council to investigate reports of human rights violation after footage appeared to show soldiers opening fire on at least a dozen civilians during a counterinsurgency operation, the New York Times reported on 15 March.

The Kasaï, in the heart of the DRC, has been plagued by deadly violence between the Congolese security forces and a local militia, Kamwina Nsapu, since August 2016.

Besides abuses attributed to Kamwina Nsapu, the UN also reported the disproportionate use of force by government security forces in their response to the situation.

The seven-minute footage, which was circulated on social media by Congolese human rights activists last month, appears to show men dressed in army uniform opening fire on a group of unarmed civilians, killing at least eight of them, including women and children.

Congolese authorities said they would carry out an investigation into video footage that appears to show elements of the army (FARDC) "summarily executing civilians, including women and children."

Commenting about the abduction of the two UN experts, a Congolese government spokesperson said judicial authorities in the area were investigating the disappearance and criticised the UN.

"It's not normal for people to come here and start moving around like this," spokesman Lambert Mende is quoted as saying. "If the government had been informed of the activities of these officials, perhaps they would have had an escort for their safety."