Nearly a week after Congo's government announced the kidnapping of two United Nations experts along with their translator and drivers, no trace of them has been found. Their abduction in a region of the country where kidnappings are rare, and where the experts were investigating abuses by state and militia forces, has raised alarm as political tensions spread over an election crisis.

Michael Sharp of the United States and Zaida Catalan of Sweden were abducted with three Congolese colleagues while traveling by motorcycle through Central Kasai province. It was not clear when exactly the kidnapping occurred.

It is the first recorded abduction of international workers in the province, a region far from the usual turmoil in eastern Congo where multiple armed groups roam.

A new report by the U.N. secretary-general has warned that violence and threats to civilians have spread to new parts of the vast country because of Congo's prolonged political crisis.

President Joseph Kabila's mandate ended in December, but he has stayed on as presidential elections once set for last year have been delayed. A political agreement reached between the ruling party and opposition after weeks of deadly protests promises an election by the end of this year and that Kabila will not run.

But the new report by U.N. chief Antonio Guterres says the agreement is in peril as the sides engage in "brinksmanship." Further delays in implementing the deal "will only serve to inflame tensions and fuel the violence that is now spreading across the country," the report says.

Parts of Congo have experienced insecurity for more than two decades since the end of the Rwandan genocide led to the presence of local and foreign armed militias, all vying for control of mineral-rich land.

But the Central Kasai region where the U.N. experts were abducted represents the new expansion of tensions.

Sharp and Catalan had been looking into recent large-scale violence and alleged human rights violations by the Congolese army and local militia groups. Hundreds of people have been killed in an upsurge of violence since July in the province, according to the U.N. Joint Human Rights Office.

While the violence is linked to local power struggles, there are also clear ties to the national political crisis, according to experts who say Congo's security forces have been known to back local leaders seen to be loyal to Kabila. Meanwhile, militia groups support those who are believed to support the opposition.

Just days after the U.N. expressed grave concern about reports of more than 100 people killed in Central Kasai region during clashes between soldiers and Kamwina Nsapu militia fighters, a video posted online appeared to show men in Congolese uniforms fatally shooting more than a dozen alleged militia members armed with little more than sticks.

International governments and rights groups have called for investigations into the shootings, which followed months of alleged violence by the militia after its leader was killed in a police operation in August.

Though little is known yet about the kidnapping of the U.N. experts, one motive might be an effort to silence independent investigators, said Ida Sawyer, Central Africa director at Human Rights Watch.

"As investigations continue, we can't rule out the possibility of involvement by the militia groups active in the area, and/or the Congolese army or government," Sawyer said.

"The disappearance ... likely will have a dramatic deterrent effect on other independent investigators looking into violence in the region," she said.

The spokesman for the U.N. peacekeeping mission in Congo, Charles Bambara, said the mission had intensified its searches for the missing experts and their colleagues.

While the U.N. experts aren't humanitarian workers, Congo is the fourth worst country in the world in terms of security incidents targeting aid workers, according to Elodie Sabau, advocacy officer with the local U.N. humanitarian office. Some aid groups have suspended work in the insecure east after multiple kidnappings.

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Associated Press writer Saleh Mwanamilongo in Kinshasa, Congo contributed.