However, the US defence attache in Colombo, Lieutenant Colonel Lawrence Smith, attended the seminar as an "observer" and his remarks there were his own "opinion", the US state department said in a statement received in Colombo.
Sri Lanka hosted the seminar, co-sponsored by its main arms supplier China, to highlight its win over the rebels after decades of ethnic conflict which according to the UN claimed up to 100,000 lives between 1972 and 2009.
Smith told the seminar that the credibility of rebel surrender claims was in doubt and he appeared to contradict a Western allegation that Sri Lankan troops killed surrendering rebels.
A key charge in a recent UN report accuses Sri Lanka of executing Tiger political leaders who are said to have worked out a surrender deal through Western diplomats, including those of the United States in Colombo.
Smith told the seminar offers of surrender came from "mouthpieces" of the Liberation Tigers of Tamil Eelam (LTTE), whose words had not demonstrated any control over the leadership.
"Their offers were suspect," Smith said, taking the floor after a retired Indian army general Ashok Metha sought clarification from Sri Lankan authorities about allegations of executing surrendering rebels.
"I think we need to examine the credibility of those offers," Smith said, in remarks which received wide coverage in pro-government media in Colombo to buttress Sri Lanka's claims that there was no surrender by rebels.
"Remarks earlier this week by the US embassy's Defense Attache... reflected his personal opinions," the US State Department statement said.
"They do not reflect the policy of the United States government."
It said the Washington "remains deeply concerned by the findings of the (UN) Secretary-General's Panel of Experts on Sri Lanka" which said there were "credible allegations" that Sri Lankan troops killed thousands of civilians.
"We believe the Sri Lankan government must act quickly and credibly to address the violations alleged in the (UN) report," the statement said.
At the end of the seminar, Sri Lanka's army chief, Lieutenant General Jagath Jayasuriya, denied that civilians were killed, but said he was open to probing alleged rights abuses in the final stages of fighting.
"I am prepared to investigate allegations, specific allegations," he said.
"I don't want to sweep anything under the rug," he said in an apparent softening of the hardline position of Sri Lanka which had insisted no civilians were killed by its troops and there was no need for an investigation.The UN has said up to 7,000 civilians were killed in the final months of fighting.