The United Nations' top human rights official voiced alarm on Sunday about rising ethnic tension and calls to violence in the Democratic Republic of Congo following disputed elections.

Massive delays and bureaucratic chaos marred the December 20 ballots to choose the president, lawmakers for national and provincial assemblies, and local councilors.

So far the election commission has only announced the result of the presidential vote -– a landslide victory for incumbent Felix Tshisekedi that the opposition has rejected as a sham.

"I am very concerned about the rise in ethnic-based hate speech and incitement to violence in the Democratic Republic of the Congo," (DRC) said U.N. High Commissioner for Human Rights Volker Turk.

He said the post-election calls for violence were particularly concerning in the eastern provinces of North and South Kivu -– which have been plagued for decades by armed groups and ethnic killings -– as well as the regions of Kasai and Katanga.

Tshisekedi hails from Kasai and Moise Katumba, one of his main rivals, from Katanga.

"Hateful, dehumanizing and inciteful rhetoric is abhorrent and can only deepen tension and violence in the DRC itself, as well as putting regional security at risk," Turk said.

He urged the authorities "to thoroughly and transparently investigate all reports of hate speech and incitement to violence and to hold those responsible to account."

Election-related tensions are common in the DRC, which has a history of authoritarian rule and violent government overthrow.

Some 250 different ethnic groups live in the vast country. It sits on considerable mineral wealth but little trickles down the population of around 100 million.