Two United Nations workers from peacekeeping mission in the Democratic Republic of Congo (DRC) have been kidnapped by unknown assailants in the central Kasai region.

The government of DRC said in a statement one of the victims was Michael Sharp, an American, and the other Zaida Catalan, a Swede. The UN lists Catalan as Chilean, however.

They had "fallen into the hands of negative forces not yet identified" near the village of Ngombe along with four Congolese nationals, including three motorcycle-taxi drivers and an interpreter, it said.

Charles-Antoine Bambara, spokesman for the MONUSCO peacekeeping mission, said on Monday the two went missing on Sunday.

Sharp and Catalan were among a UN panel of experts investigating conflicts that have simmered in DRC since the mid-1990s, when a civil war spawned dozens of armed groups and drew in half a dozen neighbouring armies.

Congolese and UN officials said efforts to locate the victims were under way.

WATCH: Can another civil war be avoided in DRC? (24:59)

An Uruguayan peacekeeper was shot and wounded last week in the same region, which has been wracked by a rebellion since September.

The uprising erupted after government forces in August killed a tribal chief and militia leader, Kamwina Nsapu, who had rebelled against President Joseph Kabila.

The violence has since spilled over to the neighbouring provinces of Kasai-Oriental and Lomami, leaving at least 400 people dead.

UN High Commissioner for Human Rights Zeid Ra'ad al-Hussein said last week that three mass graves had been discovered in the area where clashes are taking place.

The UN has almost 19,000 troops deployed in DRC, its largest and costliest peacekeeping mission. About 100 of those soldiers were recently dispatched to Kasai.

UN Secretary-General Antonio Guterres on Friday asked the Security Council to send an extra 320 UN police to the country after a deal to end a dispute over the presidential election stalled.