Nisha Biswal, assistant secretary of state for South and Central Asia, said Colombo had failed to ensure reconciliation, justice and accountability, and pressure was building for a foreign probe.
"Lack of progress in Sri Lanka has led to a great deal of frustration and scepticism in my government and in the international community," Biswal told reporters at the end of a two-day visit to Sri Lanka.
She said the US favoured a "domestic process" to investigate allegations of war crimes, including charges that thousands of civilians were killed by government forces in the final months of fighting in 2009, but there was no progress.
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"The international community's patience is wearing thin," she said, adding mounting calls for an external investigation are the result of Colombo's failure to show progress.
Britain's Prime Minister David Cameron warned during a Commonwealth summit in Colombo in November he would use London's UN position to press for an independent international investigation un