Armando Siahaan, Singapore – Muslim organizations Nahdlatul Ulama and Muhammadiyah played a significant role in the Indonesian Armed Forces' execution of alleged communists in the 1965-66 mass killings, scholars said on Thursday.

Australian historian Greg Fealy, speaking at an international conference in Singapore to discuss the murder of more than 500,000 people, claimed that documents revealed correspondence between the Army and NU leaders to prime its members for mass violence against the Indonesian Communist Party (PKI).

Although the written instructions from NU leaders to its members did not explicitly mention acts of violence, said Fealy, "they made frequent references to terms such as menumpas [eradicate or annihilate], membersihkan [cleanse], mengganyang [crush], and mengikis habis [eliminate].

"For the fervently anti-communist members of NU, [the written instructions] were an exhortation to physically eliminate all traces of communism," said Fealy, an academic from the Australian National University.

Fealy also said many kiai [clerics] played central roles in overseeing and directing the killings, and coordinated with military officers.

American anthropologist Mark Woodward said that in Yogyakarta, leaders of Muhammadiyah, the dominant Islamic group in the area at the time, issued statements declaring "destruction of the Communist Party was an individual religious obligation, not just a collective obligation.

Katharine McGregor of the University of Melbourne said that following the mass killings, NU members touted their participations as "a form of patriotic service to the nation" and reminded the New Order regime of a debt owed to the religious community.

In the wake of former President Suharto's downfall in 1998, responses from NU members regarding their role became much more divergent, according to McGregor.

In 2000, former President Abdurrahman Wahid, who was a senior member of NU, issued a personal apology to people affected by the violence and proposed to officially lift the ban on communism.

McGregor said the move met vehement opposition from senior members of NU and she gave an example of how prominent member Yusuf Hasyim wrote a letter to Islamic newspaper Republika rejecting Wahid's proposal.

But many NU members had also expressed remorse regarding the violence, she said.

During a recent interview conducted by McGregor, the current chairman of NU, Hasyim Muzadi, declined to comment on the role of NU in the 1965 violence, saying "all that happened must be considered history and not opened up again, otherwise another civil war might occur."

Separately, University of Queensland scholar Annie Pohlman, speaking to the Jakarta Globe on the sidelines of the conference, claimed women affiliated with communism suffered horrific sexual violence during the upheaval.

"There were many different forms of sexual violence, including gang rape and mass rape, but there are a whole range of different forms of sexual assault and sexual humiliation," she said.