Doctors Without Borders on Tuesday urged the forces conducting raids against rebels in northeastern DR Congo to ensure the safety of hostages believed to be held in the region.

The Paris-based charity, which goes by its French acronym MSF, expressed concern over the fate of four of its Congolese staff abducted nine months ago.

"Despite all our efforts and the fact that we have received a large amount of information, the search for our colleagues has run up against our inability to contact them, either directly or indirectly," MSF president Mego Terzian said.

"It is the same for the many Congolese who have had friends, relatives or colleagues abducted."

In a joint operation with a special UN brigade, the armed forces of the Democratic Republic of Congo have for months now been seeking to flush out some of the dozens of rebel outfits based in North Kivu.

The joint effort has recently focused on ADF-Nalu, a Ugandan Muslim rebel group which has held strongholds in the Beni area for years.

The four MSF staff were abducted in July 2013 during an ADF-Nalu attack on Kamango, in the Beni region.

The medical charity however has not been able to ascertain that the group still holds its missing employees.

The Congolese army and several civil society groups estimate that around 800 people have been taken hostage since the start of 2013 by some of the myriad rebel groups based in the restive North Kivu region.

In an earlier offensive, regular forces recaptured ADF-Nalu's bastions but the army now faces the daunting task of tracking down the rebels on some of the continent's most impenetrable terrain.

ADF-Nalu stands for Allied Democratic Forces-National Army for the Liberation of Uganda. It is one of the oldest but lesser known rebellions based in North-Kivu and is considered the only Islamist organisation in the region.

ADF-Nalu is led by Jamil Mukulu, a Christian convert to Islam, and has never really managed to take its fight against Yoweri Museveni's regime to Uganda.