A US-led resolution calling for an international probe into allegations that 40,000 civilians were killed at the end of Sri Lanka’s separatist war has been filed with the UN’s top rights body.

In a draft resolution posted on the Human Rights Commission’s website on Tuesday, the United States endorses UN human rights chief Navi Pillay’s recommendation for an external investigation into alleged war crimes in the final stages of Sri Lanka’s Tamil civil war in May 2009.

The draft welcomed Pillay’s recommendation following her visit to Sri Lanka in August that there should be an “independent and credible investigation in the absence of a credible national process with tangible results”.

International rights groups as well as UN experts have said there are “credible allegations” that up to 40,000 Tamil civilians were killed after government forces had ordered them into a no-fire zone.

The US draft noted progress in de-mining, reconstruction and re-settling war victims and effectively gave Sri Lanka another year to show results on accountability.

The US draft was backed by Britain, Montenegro, Macedonia and Mauritius.

It asked Pillay to report back on progress with an oral submission to the 27th session of the council in March next year and provide a written report by September 2015.

The draft resolution also called on Sri Lanka to investigate allegations of military excesses and expressed “serious concern” over continuing reports of human rights violations five years after the end of the decades-long separatist war.

Sri Lanka has already rejected Pillay’s call for an international probe as an “unwarranted interference”, and President Mahinda Rajapakse has accused Washington of treating Colombo like Muhammad Ali’s “punching bag”.