International and local human rights activists condemned an attack on an Indonesian watchdog group over its opposition to the military offensive in Aceh.

About 100 uniformed members of a nationalist youth group on Tuesday vandalised the offices of the Commission for Missing Persons and Victims of Violence (Kontras) and slightly injured five staff, media reports said Wednesday.

The attack on Kontras was prompted by its criticism of a continuing military operation against Aceh separatist rebels, according to group members quoted by the Jakarta Post and Antara news agency.

Kontras's founder Munir, quoted by Antara, said youths were yelling that statements by himself and the watchdog on Aceh were unpatriotic. Witnesses said the attackers were from a group called Pemuda Panca Marga.

Kontras official Orie Rachman, quoted by the Jakarta Post, said the group also visited the Kontras offices Monday to make threats. "Strangely, no police officers were present [on Tuesday] even after we had reported their threat," he said.

New York-based Human Rights Watch, in a statement, charged that police failed to stop the attacks. "The failure of the police to respond to these attacks on a leading human rights organisation sets a bad precedent for all groups working on Aceh," said Brad Adams, executive director of the group's Asia division.

Several local rights campaigners also denounced the attacks. Asmara Nababan was quoted by Antara as calling the incidents a clear example of fascism which could threaten democratisation.

Rights campaigners have frequently been targeted in Indonesia in recent years despite a climate of freer expression since the 1998 departure of dictator Suharto.