A Burundian rebel group says Tanzanian authorities have arrested four of its top leaders.

The Popular Forces of Burundi said those arrested Saturday include the rebel group’s commander in chief, Maj. Gen. Jérémie Ntiranyibagira.

Human rights activists condemned Tanzania’s decision to make the arrests, saying the Burundians face possible execution upon deportation to Burundi.

Pacifique Nininahazwe, president of the human rights group known as FOCODE, said the arrests taint Tanzania’s image as a neutral mediator in Burundi’s peace process.

It was not immediately possible to get a comment from the Tanzanian government and a spokesman for Burundi’s presidency said the president has not been briefed.

Burundi has faced sporadic violence since President Pierre Nkurunziza announced plans in April 2015 to seek a disputed third term that he ultimately won.