Rwanda's government paid contract killers to assassinate opponents and critics of President Paul Kagame, according to a Canadian newspaper investigation published on Saturday.

The Globe and Mail spoke to exiles based in South Africa and Belgium who claim to have been recruited by Rwanda's military intelligence to silence critics of the strongman's regime.

Top United Nations rights officials vowed Wednesday to do everything in their power to prevent conflict-wracked South Sudan from sliding into genocide, and warned the warring factions they would be held responsible if famine breaks out.

Firing off a damning attack against South Sudan's President Salva Kiir and rebel leader Riek Machar, the U.N.'s High Commissioner for Human Rights Navi Pillay said she was "appalled by the apparent lack of concern about the risk of famine displayed by both leaders."

U.S. Secretary of State John Kerry left late Tuesday on his first major tour of Africa focused on some of the continent's most brutal wars including the bloodshed in South Sudan. 

The trip, which will take in Ethiopia, the Democratic Republic of Congo and Angola, will seek to "encourage democratic development, promote respect for human rights, advance peace and security," State Department spokeswoman Jen Psaki has said.

Rwanda on Wednesday warned not enough was being done to prevent future mass atrocities as the United Nations passed a resolution pledging to heed the lessons from the country's 1994 genocide.

The U.N. Security Council adopted a resolution solemnly calling on states "to recommit to prevent and fight against genocide and other serious crimes against international law."

Rwandan police on Tuesday paraded four people accused of planning grenade attacks on the country's leaders in revenge for the assassination of a former spy chief and fierce critic of President Paul Kagame.

Kigali's former intelligence chief Patrick Karegeya, who lived in exile in South Africa, was found strangled to death in a Johannesburg luxury hotel on New Year's Day.

Police in Rwanda announced Monday they have arrested three people on charges of threatening state security, among them a radio journalist previously reported missing and a peace activist musician.

Rwandan police said in a statement that the three were accused of connections with the Rwanda National Congress (RNC), an opposition party in exile linked to Patrick Karegeya, a former spy chief who was murdered in South Africa at the beginning of the year.

A Norwegian court on Tuesday authorized the extradition of a man suspected of being involved in the Rwandan genocide two decades ago.

Eugene Nkuranyabahizi, 41, was arrested in Norway in May last year but has lived in the country since 1999.

France's new Prime Minister Manuel Valls on Tuesday said his country could in no way be held responsible for the Rwandan genocide 20 years ago.

His comments came amid a bitter spat over allegations by Kigali of Paris's complicity in the bloodshed.

Rwandan President Paul Kagame took a thinly-veiled swipe at France on Monday, saying it was impossible to "change the facts" about the genocide 20 years ago.

"The passage of time should not obscure the facts, lessen the responsibility, or turn victims into villains," he said in a speech during commemorations marking the 20th anniversary of the genocide.

Rwanda on Monday held solemn commemorations marking the 20th anniversary of the genocide, with many survivors overcome with grief as they relived the trauma of the massacres that left nearly a million dead.

The events also bore reminders of festering anger, as a major diplomatic row broke out over renewed allegations of French complicity in the genocide.