At the same time, he reaffirmed U.S. military ties to the Cambodian government, The New York Times reported.
After meeting with Gen. Tea Banh, Cambodia's defense minister, Panetta said he wanted to emphasize that the United Stated supported "the protection of human rights, of civilian oversight of the military, of respect for the rule of law and for the right of full and fair participation in the political process."
In a report this week, Human Rights Watch charged that Cambodia had killed numerous labor leaders, journalists and opposition over the past 20 years.
Panetta was in Cambodia as part of a tour intended to bolster U.S. relationships and counter China's growing influence in the region, officials said.
U.S. Special Forces units are training Cambodian troops in counter-terrorism.