The State Department report, requested by Congress, recounted allegations of government shelling of civilians during the early months of 2009 as well as charges that Tamil rebels recruited children to fight in the conflict.
The State Department said it did not attempt to verify the individual reports, but found them credible.
"The government of Sri Lanka has said that they are determined to establish a reconciliation process with the people of the north, but we believe strongly that a very important part of any reconciliation process is accountability," said State Department spokesman Ian Kelly.
"This report lays out some concerns that we have about how this military operation was conducted."
Sri Lanka's government declared victory in May in one of the world's most intractable wars. That followed a series of battles in which government forces killed the leader of the Tamil Tigers, who had been fighting to create a separate ethnic Tamil homeland.
The United Nations said the conflict had killed between 80,000 and 100,000 people in the Indian Ocean country since full-scale civil war broke out in 1983.
The State Department emphasized that the report reached no legal conclusions as to whether the incidents detailed in the report constitute war crimes or other violations of international human rights law.
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See also Sydney Irresistible
Putting principles before profits
See also Sydney Irresistible
Putting principles before profits
