JAKARTA (Reuters) - The U.S. government had intimate knowledge of the mass killing of alleged communists in Indonesia in the mid 1960s even as it failed to publicly reveal the slaughter, newly declassified U.S. documents show.The documents also reveal that Indonesian army intermediaries told Western embassies they were considering toppling then president Sukarno less than a fortnight after the killing of six generals by rebel military personnel that sparked the bloodletting.

The murder of the generals on Sept. 30, 1965, is still widely depicted as an attempted communist coup against Sukarno.

The murders were used as a pretext for an anti-communist pogrom by Indonesia’s military and Islamic groups that led to at least 500,000 deaths.

One of the worst massacres of the 20th century, the killings in 1965 and 1966 have never been officially investigated and perpetrators have never faced justice.

Indonesia, the world’s largest Muslim-majority country, still has periodic bouts of anti-communist hysteria. Last month, a meeting by human rights activists and victims of the anti-communist purge was shut down after Islamists and nationalists rioted outside the venue.

One cable from the U.S. embassy in Jakarta to the Department of State, written three months after members of the communist party, the PKI, were first targeted, said there were “an estimated 100,000 PKI deaths”.

In Medan, on the island of Sumatra, clerics from the Muslim group Muhammadiyah urged members to kill communists, according to a telegram from the U.S. consul.”‘Conscious’ PKI members are classified as lowest order of infidel, the shedding of whose blood is comparable to killing chicken,” the consul said in one report.

Muhammadiyah youth group chairman Dahnil Anzar said the documents showed “nothing new” but such conflict should not happen again.

A U.S. consular officer in the city of Surabaya, in East Java, said in a telegram members of Ansor, the youth wing of the Nahdlatul Ulama Islamic organization, were responsible for “widespread slaughter” and they thought killing communists was a “ticket to heaven”.

“Army is quietly releasing nightly 10 to 15 prisoners to Muslims for execution,” the official in Surabaya said in another dispatch.

Abdul Rochman, secretary general of Ansor, told Reuters that the group’s involvement in the killings of 1965 was a “form of self-defense”.

“(The PKI) carried out provocative activities like aggressive recruitment and went further by insulting our imams, poisoning and killing our members, burning mosques,” he said.”We believe the NU’s reaction was relatively measured and not disproportionate.”Indonesia’s chief security minister, Wiranto, declined to answer questions. Army deputy chief of staff, H Siburian, said he had not seen the documents so could not comment.

CONCERN ABOUT LEFT-WING POLICIES

Rochman, from NU’s Ansor, said he supported any move for dialogue and reconciliation.

“We must make sure all voices are heard - NU’s voice, the victims’ voices, the children of suspected communists.”

Phelim Kine, deputy Asia director of Human Rights Watch, said redress for victims was “long overdue”.

The U.S. embassy did not respond to requests for comment.

While the documents do not solve the mystery of why the six generals were killed on Sept. 30, they do show that the United States and its Indonesian military allies were deeply concerned about Sukarno’s left-wing policies and ties to the communists.

Then U.S. ambassador Marshall Green wrote on Oct. 12, 1965, that the army had approached western embassies through an “intermediary” and said they were considering a “quick move” to overthrow Sukarno, although no final decision was made.

In the event of such a coup, Green says the United States could help with “anything from covert operations and assistance [to] transport, money, communications equipment, or arms”.

In January 1967, Major General Sjarif Thajeb told a U.S. embassy official Sukarno planned to accuse foreigners and their “little army friends” of orchestrating the Sept. 30 killings. The president’s plan had galvanized the military “hawks” to oust Sukarno “by March”.

Army general Suharto became acting president in March, ushering in three decades of military dictatorship.