The main target of the operation is the RED-Tabara, the Burundi Human Rights Initiative said, referring to the most active of the rebel groups which is deemed a terrorist organisation by the Burundian government.

Burundi has always denied carrying out any secret operations, insisting it has acted only within the framework of joint operations by the East African Community (EAC), African Union or United Nations.

"The Burundi National Defence Force is a modern and professional army whose interventions are all carried out legally," Defence Minister Alain Tribert Mutabazi told reporters on Tuesday.

He said those who claimed there were Burundian forces on Congolese soil had a "hidden agenda" but added that the army was preparing to intervene as part of the regional force.

The EAC agreed in June to deploy troops to fight the myriad rebel groups involved in an upsurge of violence in the eastern DRC that has ensnared neighbouring countries.

"Several hundred Burundian soldiers and Imbonerakure -- more than 1,000 -- are believed to have gone to the DRC in successive waves since late 2021," the BHRI said in a report.

The Imbonerakure are members of the youth league of the ruling CNDD-FDD party of President Evariste Ndayishimiye.

- 'Angry' about treatment in DRC -

"For more than 10 years, Burundian soldiers and Imbonerakure have periodically sought to hunt down Burundian armed opposition groups in the DRC," the BHRI said.

"But the current operation is different in scale and duration," it said, adding that about 700 were estimated to be on DRC soil in the early phase of the deployment in December.

The rights group, which is based abroad, said it collected testimonies from soldiers, relatives and members of the ruling and opposition parties.

It said it was not able to confirm the exact numbers of troops or incursions, although it reported that the UN Group of Experts collected information on 17 incursions in the Uvira region between September last year and this March.

"Some soldiers are ordered to swap their military uniforms for civilian clothes and leave behind possessions that could identify them," said the BHRI.

The report said those returning have been warned not to talk about their mission, and little or no explanation is given to the families of those who die on the battlefield.

It said some Imbonerakure have become "angry" about their treatment during the military operation, with some saying they felt they had been deceived or abandoned.

In May, Ndayishimiye said he was ready to hold a dialogue with Burundian rebels in DRC, in particular RED-Tabara and the National Forces of Liberation (FNL).

Founded in 2011, RED-Tabara has been accused of a string of attacks in Burundi since 2015.

In September it claimed responsibility for an attack on the international airport in Bujumbura, the country's economic capital.