"Human Rights Council member and observer countries that backed the October 2015 UNHRC resolution, should make clear that foreign participation in a war crimes tribunal was already decided by the council and is not subject to renegotiation," said HRW Asia director Brad Adams.
The Sri Lankan government sought international involvement to ensure justice and accountability "so there is no excuse for backtracking now," Adams said.
Criticising Sirisena for his remarks, the HRW said, "the President needs to understand that international participation in a war crimes tribunal was not a vague promise to the UN but a firm commitment to the thousands of Sri Lankans who suffered during the country's long civil war."
President Sirisena's statement came just weeks before a scheduled visit to the country by the UN High Commissioner for Human Rights, Zeid Ra'ad Al Hussein.
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The HRW said the real rights gains made by the Sirisena administration will rapidly fade if the families of wartime victims feel that their one hope for justice was dropped on the basis of political calculations.
In October 2015, Sri Lanka co-sponsored a UN Human Rights Council resolution calling for a special judicial mechanism to prosecute war crimes to be established - with support from Commonwealth and foreign judges, prosecutors and investigators.
The final months of the 26-year war were the most bloody, with the government accused of repeatedly shelling safe zones set up to protect civilians.