Rights groups are urging the U.S. to release secret files on Indonesia's anti-communist massacres of 1965-66, as the Southeast Asian country takes a tentative step toward a reckoning with one of the worst atrocities of the last century.

The push by Human Rights Watch and Indonesian group Kontras comes ahead of a conference in Indonesia next week that will be a rare public discussion of the events of 50 years ago.

Human Rights Watch Executive Director Kenneth Roth said Wednesday the massacres were "one of the most horrendous crimes of our era."

There is no official figure for the number of people killed but researchers estimate half a million.

Indonesia's human rights commission last month asked the U.S. to declassify files that could show how the killings were planned.