US State Department war crimes investigator Stephen Rapp "listened to eyewitness accounts about serious human rights abuses" during his five-day mission to the island that ended on Saturday, the embassy said.
"During ambassador Rapp's discussions, he listened to eyewitness accounts about serious human rights abuses and violations of international humanitarian law, including those that occurred at the end of the war," the embassy said in a statement.
"In that context the government of the US encourages the government of Sri Lanka to seek the truth through independent and credible investigations, and where relevant, have prosecutions," it said.
During the visit, the embassy sparked fury within the Sri Lankan government by tweeting that army shelling killed hundreds of families in January 2009, the final stages of Sri Lanka's war against Tamil rebels.
A senior Sri Lankan foreign ministry official said the comments were discussed when Rapp called on Foreign Minister G. L. Peiris on Friday