A Congolese journalist is being held for questioning by police over the 2017 killing of two UN experts in the central Democratic Republic of Congo, the military prosecutor's office said on Tuesday.

Police arrested Sosthene Kambidi in his hotel room in the capital Kinshasa on Monday evening after his name was mentioned by two witnesses in the case, a senior prosecutor told AFP, declining to be identified.

About 30 people have been accused of taking part in the March 2017 kidnap and murder of American Michael Sharp and Swedish-Chilean Zaida Catalan, hired by the United Nations to probe violence in the Kasai region of central DRC.

The trial that began in June 2017 has been stalling, however, and the UN has appointed a judicial expert to assist.

A video purporting to show the two being murdered by the Kamuina Nsapu militia -- an ethnically based rebel group centred on a local tribal leader -- was broadcast on state television in order to quell rumours the army was implicated in the deaths.

"The journalist Sosthene Kambidi is an important informant because he was among the first people who held the video of the murder of Zaida Catalan and Michael Sharp," a senior Congolese military prosecutor told AFP.

Kambidi, who has worked as a freelancer for AFP and at the Congolese online media Actualite.cd in Kananga, capital of Kasai Central province, was "questioned by the United Nations' monitoring mechanism of the attacks as a witness because his name was mentioned by two other witnesses," the prosecutor said.

The journalist must reveal "who gave him the video, how he got it," the prosecutor said. "Was he a direct witness? Was he in contact with those who killed the two experts? Was he an actor himself? We need answers."

Asked what the military prosecutor's office will do if Kambidi refuses to turn over his sources, the prosecutor declined to comment.

According to the official version from the authorities at the time, Sharp and Catalan were killed by militiamen of the Kamuina Nsapu sect, who were at war with the regular army.

The conflict left 3,400 people dead and tens of thousands displaced between September 2016 and mid-2017 in the Kasai region.

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