In the toughest warning since the end of fighting in May 2009, US Assistant Secretary of State for South and Central Asian Affairs, Robert Blake, said Sri Lanka risked a forced international investigation.
His remarks came as the UN Human Rights Council was meeting in Geneva, where Sri Lanka has previously managed to avoid condemnation thanks to the backing of several nations, including strong allies China and Russia.
Blake, a former ambassador to Sri Lanka, said it was "preferable" for Sri Lanka to have its own investigation in line with internationally accepted human rights standards, rather than face an external inquiry.
"It's important to say that if Sri Lanka is not willing to meet international standards regarding these matters, there would be pressure to appoint an international commission to look into these things," Blake told AFP.
Sri Lanka has refused to investigate alleged war crimes, but President Mahinda Rajapakse has appointed a panel to probe why a 2002 ceasefire b