The 186 alleged rebels were captured near the two countries' border in 2015 and early 2016 amid violence that broke out after Burundian President Pierre Nkurunziza announced plans to seek a third term, a move his opponents said violated the constitution.

"We are finalizing plans to extradite them," Congo's government spokesman Lambert Mende told Reuters.

He did not know how long the process would take.

"We cannot accept that Congo serve as a base to launch hostile actions against a member country of the ICGLR," he said, referring to regional body, the International Conference on the Great Lakes.

At least 450 people have died since 2015 in Burundi in clashes between protesters and security forces, revenge killings and a failed coup, stoking fears of wider unrest in a region still haunted by neighboring Rwanda's 1994 genocide.

U.N. experts said last year that some of the alleged rebels captured in Congo confirmed receiving military training or other support from Rwanda. Kigali has denied those charges.

The United Nations and rights groups have accused Burundian forces of committing gross human rights violations and possibly crimes against humanity, including summary executions, targeted assassinations, arbitrary detention, torture and sexual violence - charges the government denies.

"These 186 presumed rebels risk being subjected to grave violations of human rights," Florent Geel, Africa director for Paris-based International Federation for Human Rights (FIDH), told Reuters.

"Tribunals in Burundi today are not impartial... There is a systematic practice of torture...There are extra-judicial executions," he added.

A Burundi government spokeman said he had no information on the extraditions.