Up to 40,000 Tamil civilians were killed at the end of the separatist war in 2009, rights groups and experts have said.
In a draft resolution yesterday, the US endorses UNHRC Navi Pillay's recommendation for an external probe into the alleged war crimes.
The US, Britain, Montenegro, Macedonia and Mauritius had jointly tabled the draft resolution at the UNHRC session that began yesterday.
The resolution welcomes Pillay's "recommendations and conclusions on the need for an independent and credible international investigation in the absence of a credible national process with tangible results".
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The resolution, which is to be voted at the end of this month, will be the third in as many years on Sri Lanka's accountability and reconciliation with the Tamil minority.
The resolution also recommends the establishment of a truth seeking mechanism, a national reparations policy, to hold individuals responsible accountable for violations of international human rights law, investigate all attacks against religious minorities, journalists, human rights defenders and civil society.
It also seeks meaningful devolution of powers in terms of the 13th amendment to the constitution and to empower the Tamil chief minister in the north with necessary resources and authority.
External Affairs Minister GL Peiris is to make his submission in Geneva tomorrow, the government sources said.
Sri Lanka has launched an extensive lobbying campaign against the US moved resolution.
Sri Lanka's arguments are based on unfair targeting by the US and Britain at the behest of the pro-LTTE diaspora, the international community needs to allow time and space for Sri Lanka to achieve reconciliation in recognition of the progress made since the end of a 30-year-old conflict.