More than 1,000 Burundian troops have been covertly deployed in eastern Democratic Republic of Congo (DRC) since September, wearing the uniform of Congolese troops and working alongside them in the fight against M23 rebels, an unpublished UN report seen by Reuters shows.

Citing security and intelligence sources and sources close to Congolese army command, the report by the UN Group of Experts in Congo said the troops were airlifted from Burundi to eastern DRC by Congolese army planes starting from September 21.

The Burundian and DRC governments and the Congolese army did not immediately reply to a request for comment from Reuters.

The report was shared internally with UN Security Council members on Dec. 15 and seen by Reuters on Saturday. It is due to be published in January.

It sheds light on DRC's security strategy in the east beyond the known deployment of UN peacekeepers and Congolese troops, who were backed up by an East African Community (EAC) regional force until earlier in December.

The report said the Burundian government had denied to the UN experts any Burundian troop deployment outside the EAC arrangement. It also said DRC military authorities had told the group they were unaware of the cooperation with Burundian troops highlighted in the report.

Citing its sources, the report said the Burundian troops were deployed outside the EAC arrangement and alongside Congolese troops and allied armed groups fighting M23 in North Kivu province.

Since 2022 the M23 offensive there has worsened the region's decades-long security and humanitarian crisis, forcing about 1-million people to flee their homes.

DRC, Western powers and the UN expert group say the Tutsi-led rebel group is supported by neighbouring Rwanda. Rwanda denies all involvement, but the accusations have led to a major diplomatic crisis in the region.