Six organisations in a joint letter expressed alarm over alleged intimidation, threats and reprisals against those likely to engage investigators probing the bloody finale to Sri Lanka's Tamil separatists war.
The letter published Tuesday on the website of the Geneva-based International Commission of Jurists urged "meaningful steps" to protect those who cooperated with the war crimes probe which was approved by the UN Human Rights Council in March.
"Human rights defenders in Sri Lanka face widespread intimidation," the letter said adding that it could intensify ahead of the presentation of an update on the investigation at the council's September and March sessions.
Sri Lanka has made it clear that it will not grant visas to UN investigators.
Colombo says it will also not accept the authority of the UN Human Rights Council to probe charges that Sri Lanka's military killed 40,000 civilians in the final months of the war in 2009.
The open letter, also signed by Amnesty International,