US diplomat Robert Blake said Colombo should carry out its own investigation into allegations contained in a United Nations report of atrocities during the 2009 offensive against Tamil Tiger rebels.
"The international mechanism can become appropriate in case of states that are unable or unwilling to meet their obligations," the assistant secretary of state for south and central Asia told reporters at the end of a two-day visit to Sri Lanka.
Colombo has rejected a report commissioned by UN Secretary-General Ban Ki-moon which found "credible allegations" that the Sri Lankan army and Tamil Tigers rebels were guilty of war crimes or crimes against humanity.
The UN has estimated that tens of thousands of people died in the 2009 offensive, in which the government killed the Tamil Tigers' leaders, ending a 37-year separatist campaign.
Blake, a former ambassador to Sri Lanka, said the island could have its own investigation in line with internationally accepted human rights standards, rather t