JAKARTA (REUTERS) - The head of an Indonesian group representing victims of the mass killings of alleged communists in the mid-1960s said he had pinpointed 16 suspected mass grave sites in central Java that could contain about 5,000 bodies.

Mr Bedjo Untung of the 1965 Murder Victims Research Foundation (YPKP 65) said the sites were around the town of Purwodadi, where suspected communists were sent for execution.

Mr Bedjo has asked the human rights commission (Komnas HAM) to investigate.

"Some of the bodies were thrown into a ravine. Some were thrown in wells," Mr Bedjo told a new briefing, citing the testimony of three witnesses.

"I have a witness who when he was a kid used to follow his father to wash the dead bodies. Every morning, people sent them around 50 bodies. So we estimate there were around 5,000," he said, adding that human bones had been found in the area.

"I can conclude that they were suspected followers of the Indonesian communist party (PKI)," said Mr Bedjo, who was himself jailed for being a member of the now banned PKI.

One of the worst massacres of the 20th century, some historians say at least 500,000 people died in an orgy of bloodshed after then-general Suharto took power following an abortive coup.

A million or more people were jailed, suspected of being communists. The killings in 1965 and 1966 have never been officially investigated and perpetrators have never faced justice despite a pledge by President Joko Widodo to resolve human rights issues.

Mr Bedjo said that 122 other suspected sites of mass graves had previously been identified.

According to newly declassified US documents, the US government had intimate knowledge of the mass killing of alleged communists even as it failed to publicly reveal the slaughter.

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.

Mr Amiruddin Al Rahab, a commissioner at Komnas HAM, said he would look into the matter.

The rights group has previously made recommendations to launch an official investigation and set up ad hoc human rights courts but so far these have not been enacted.

Indonesia, the world's largest Muslim-majority country, still has periodic bouts of anti-communist hysteria.

In September, a meeting by human rights activists and YPKP 65 was shut down after Islamists and nationalists rioted outside the venue.