News agency Reuters reports that the kidnappers are "unknown assailants" and that it is unclear when the kidnapping took place.
In a statement, the Congolese government said that the UN workers had "fallen into the hands of negative forces not yet identified". They and the four Congolese citizens were last seen in the village of Ngombe in Congo's Kasai Central province.
"The administrative and security services are working ... In concert with Monusco (the UN mission) to obtain the liberation of the kidnapped persons," said the government statement, which was signed by Information Minister Lambert Mende.
A spokesperson for the Swedish Ministry for Foreign Affairs has confirmed that a Swedish national is missing in the Democratic Republic of Congo. He stressed that the case is at a very sensitive stage and did not want to confirm whether it concerns a kidnapping. However, he did confirm that the Swedish national is working in Congo as part of a UN mission.