Congo's government says two United Nations officials, one American and one Swedish, have been kidnapped along with four Congolese citizens in Kasai Central province.

Government spokesman Lambert Mende said Monday that Michael Sharp of the United States and Zaida Catalan, a dual Chilean-Swedish citizen, were taken with three Congolese drivers and a translator while travelling by motorcycle.

A UN spokesman said they were missing and that UN peacekeepers were searching for them, without giving further details. The government statement gave no date for the incident, which occurred on a bridge near the village of Ngombe.

Judicial authorities in the province have opened an investigation and are working with the UN mission in Congo to free those held.

"The administrative and security services are working ... in concert with Monusco (the UN mission) to obtain the liberation of the kidnapped persons," the government statement signed by Information Minister Lambert Mende added.

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

Deadly clashes in region since July

The vast Central African nation is home to multiple militias competing for stakes in its rich mineral resources.

Kasai Central province, in remote, heavily forested central Congo, has been riven by clashes between security forces and a local tribal militia called the Kamuina Nsapu since July.

At least 400 people have been killed and 200,000 have been displaced since the fighting broke out when police killed the militia's leader last August, the UN mission says.

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 the clashes are taking place.

Security across Congo has worsened since President Joseph Kabila failed to step down when his mandate expired in December.