– Two United Nations officials and four Congolese citizens have disappeared in a conflict-ridden region of the Democratic Republic of Congo where army soldiers have been accused of killing civilians, the U.N. mission there has said.

The officials — Michael Sharp, an American, and Zahida Katalan, a Swede — were traveling in the Kasai region on Sunday with three Congolese drivers and a translator when they disappeared, the U.N. mission said on Monday. It added that it was doing everything possible to locate them.

The officials, who are in Congo as part of a peacekeeping mission, were investigating possible human rights violations after reports that government soldiers had killed at least a dozen unarmed civilians, including children. Videos emerged on social networks showing what appeared to be government soldiers walking down a road and shooting people. The European Union, the United Nations and the United States called on the Congolese government to investigate the footage, which rights activists say is evidence of war crimes.

The Congolese government said that the U.N. officials had traveled by motorcycle and were thought to have been abducted by unidentified forces near the village of Ngombe in the Bukonde area.