Reporters Without Borders (RSF) on Thursday called for the release of four journalists and their driver who were arrested while covering a deadly clash between Burundi security forces and rebels from neighbouring DRCongo.
Burundi police said 14 members of the group RED-Tabara were killed on Tuesday.
The organisation, based in the east of Democratic Republic of Congo, is headed by one of Burundi's most outspoken opponents, Alexis Sinduhij, the government and diplomats believe.
RSF said the journalists with the Iwacu newspaper "were arrested (Tuesday) at midday while trying to get witness statements from residents fleeing the fighting".
The group said the reporters were being held alongside their driver in Bubanza in northwestern Burundi.
"These journalists were just doing their job by going to the scene to verify information about armed clashes... we urge authorities to free them without delay," said the RSF's Africa chief, Arnaud Froger.
Police spokesman Moise Nkurunziza did not want to reveal the reason behind their arrest during a press conference, citing "confidentiality of investigations".
Human Rights Watch (HRW) on Wednesday also demanded the journalists' "immediate release."
A Burundian journalist, speaking on condition of anonymity, said "the objective was to prevent the presence of the media in this area, and it was successful, no other information aside from that given by officials is getting out".
A local official in Bubanza indicated the situation was still tense and told AFP a police officer had been killed by a "residual group of rebels" on Wednesday evening.
Iwacu -- one of the last independent publications in the country -- has previously reported on cases of extrajudicial killings and arbitrary arrests during attacks in this area of Burundi.
RSF recently warned that there was such a crackdown on the media in Burundi that "there is a risk of all forms of independent journalism disappearing less than a year before the presidential election of May 20, 2020."
Burundi is currently ranked 159th out of 180 countries by RSF's world press freedom index.