KINSHASA, Sept 24 (Reuters) - Rights groups called on military authorities in Democratic Republic of Congo on Friday to release a journalist arrested on terrorism charges for the possession of a video showing the assassination of two U.N. sanctions monitors in 2017. Sosthene Kambidi, who works for Congolese news site Actualite.cd and sometimes with international news agencies, was arrested by the army prosecutor at a hotel in Kinshasa on Monday night, he said in a WhatsApp message to a Radio France International (RFI) journalist, which was shared with Reuters. Kambidi contributed to an investigation by RFI and Reuters in December 2017 which revealed that state security agents had helped plan a trip by the two U.N. monitors to investigate reports of atrocities during an armed conflict in a rural part of Congo"s Kasai region. Kambidi, who also accompanied Reuters and RFI journalists on a March 2017 reporting trip that revealed the existence of several mass graves in Kasai, is being prosecuted by the military for "criminal conspiracy, rebellion and terrorism", his lawyer Gode Kabongo told Reuters by telephone. Amnesty International, Human Rights Watch and Reporters Without Borders (RSF) all called for Kambidi to be released. Denied access to his family or a lawyer for 36 hours after his arrest, Kambidi was questioned over how he acquired footage of the killing in 2017, said Jean-Mobert Senga, an Amnesty International researcher. "Until proven otherwise, Kambidi is a journalist who has done nothing but his job, and should not be forced to reveal his sources," Senga told Reuters. A government spokesman did not immediately respond to a request for comment on Kambidi"s arrest. The army could not be reached for comment. During the 2017 U.N. mission, the two experts, Zaida Catalan, a Swede, and Michael Sharp, an American, were stopped along the road by armed men, marched into a field and executed. Congolese authorities initially blamed a militia, arrested about two dozen alleged fighters and charged them with involvement in the killings. The government later said it could not exclude the possibility that state agents were involved. They have, however, denied suggestions by rights groups that higher-level government and security officials might have been involved in the killings.