The UN human rights office called for a credible investigation into the shooting of an opposition journalist, noting that in the past such investigations have faltered.

The Office of the High Commissioner for Human Rights called on the government to "conduct a prompt, thorough and credible investigation" into the murder of Khim Sambor and his son, who were gunned down Friday night on a street in Phnom Penh .

The murder two weeks ahead of a national election, "irrespective of the motive," could revive in people's minds "the fear of politics," the UN office said.

Ministry of Interior spokesman Khieu Sopheak said Thursday the investigation was continuing, but no motive has been established.

"The investigation is not remaining in darkness," he said.

The UN rights office said in its statement Thursday a credible investigation was warranted "in the context of the continued impunity for past killings of journalists."

Friday's killing follows the murder or attempted murder of at least eight journalists since 1994, the rights office said, "all cases in which the perpetrators continue to enjoy impunity."

Human rights groups told reporters Thursday "the assassinations of reporters and other opposition political activists causes weakness of the general situation of peace and causes fear among voters."