Andrie Yunus, deputy coordinator of the KontraS rights group, suffered serious injuries when two men on a scooter threw acid at him this month while he was riding a motorbike, right after he finished recording a podcast about the military's influence in the Indonesian government.

Four members of the military's Strategic Intelligence Agency (BAIS) were arrested, and remain in custody.

Police separately announced the arrest of two other suspects, prompting questions on the links, if any, between the six suspects -- something the authorities have yet to clarify.

BAIS head Yudi Abrimantyo stepped down on Wednesday in a move military spokesman Aulia Dwi Nasrullah described as a "form of responsibility".

The spokesman did not give further details or specify who would replace Yudi.

UN rights chief Volker Turk has condemned the attack on Andrie as a "cowardly act of violence" and called for a full investigation.

On Thursday, Amnesty International Indonesia said the case was "taking on political undertones" with "conflicting facts" emerging from police and military probes, which appear to be separate.

It urged President Prabowo Subianto to launch an independent investigation and for the case to be referred to a civilian court.

Amnesty director Usman Hamid told AFP it was unclear what Yudi's resignation meant or where the investigation was headed next.

The Andrie case "is a systematic terror designed to silence critical voices and the constitutional rights of citizens to express their opinions," Usman said in a statement.

Prabowo, an ex-general in office since 2024, has described the attack as "terrorism" and has vowed to "uphold the law" regardless of who was responsible.

Indonesia has undergone "further democratic backsliding, crackdowns on protests, media censorship, and intimidation of activists" under Prabowo, Human Rights Watch said in its latest report on the country.