Assistant Secretary of State Nisha Biswal stopped short of endorsing a deadline set last month by British Prime Minister David Cameron, who said he would call for a UN-backed inquiry into allegations of war crimes unless there was progress on postwar reconciliation by March.
A UN report has suggested Sri Lanka's military may have killed up to 40,000 civilians in the final months of the war in 2009 as it crushed ethnic Tamil rebels, also accused of atrocities.
"We would like to see Sri Lanka address these issues through its own processes and we hope that can in fact be the case," Biswal told reporters, adding that recommendations of a Sri Lankan-government appointed reconciliation commission pointed the way forward.
She said without real progress, the patience of the international community "will start to wear thin."
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The government of President Mahinda Rajapaksa has so far ignored calls for a thorough local inquiry into war abuses and says it will not allow any international probe. He denies allegations of abuses by the army.
Biswal, whose brief covers South and Central Asia, also voiced concern Friday about political violence in Bangladesh, where street clashes between rival factions have killed about 40 people and wounded hundreds in the past month.
The opposition is resisting government plans to hold a general election on Jan. 5 unless Prime Minister Sheikh Hasina allows a neutral caretaker administration to oversee the polls.
She said failure to achieve a peaceful political transition poses the greatest obstacle to Bangladesh continuing its progress of the past decade in areas such as reducing child and maternal mortality, and improving food security.
Analysts fear the political chaos could exacerbate the economic woes of the country of 160 million people and lead to radicalization in a strategic pocket of South Asia.