The United Nations has spoken out against the detention of a radio station executive in Burundi for broadcasting the confession of a man who claimed responsibility for the murder of three Italian nuns in September last year and refusing to reveal the source's identity.

According to media outfit Expatica, the confession allegedly mentioned the name of a top official in the small land-locked Central African country, which is due to hold elections in May and June.

The three Catholic nuns, who were between 75 and 83 years of age, were murdered in Burundi's capital Bujumbura. Although an official police report into the murder was released, a man called up Bob Rugurika of the independent African Public Radio and apparently confessed his involvement in the crime.

Rugurika subsequently refused to name the man to authorities, and he was arrested last month for duplicity in the murder case.

Burundi law currently requires members of the media to reveal their sources when reporting on "matters of state security and public order."

Rugurika attempted to post bail, but a judge last week denied his motion.

"No one should be prosecuted for legitimately exercising his fundamental right to freedom of expression and opinion," spokeswoman for the UN human rights office Ravina Shamdasani told Expatica in Geneva. She also said the case raises  "serious concerns about the exercise of the freedom of expression in Burundi, particularly ahead of the upcoming elections."

Shamdasani criticised the vague media law in Burundi, stating that the legislation could easily be circumvented to justify violations of freedoms of expression in Burundi. It is not the first time the UN has urged the Burundi goverment to review the law.

Shamdasani said that the United Nations will monitor Rugurika's case.

According to Expatica, critics of the government in Burundi view this action as part of an effort to "sideline political challengers ahead of the elections."