Jendayi Frazer, the assistant secretary for African affairs, used her secret visit for talks with the transitional government. She called for "dialogue and reconciliation," excluding jihadists "who are intent on making Somalia ungovernable," the BBC said.
Frazer visited Baidoa, not the capital, Mogadishu, where the Ethiopian troops backing the transitional government have been battling Islamist forces for the past two weeks. Her trip was kept secret until she was out of the country.
Mogadishu was quiet Saturday as a truce appeared to be effective, The New York Times reported.
Human rights groups have accused Ethiopian soldiers of shelling civilian neighborhoods during the fighting. About 1,000 people have been killed or hurt and 100,000 turned out of their homes.
Frazer acknowledged that the Ethiopians are not blameless.
"Everybody used excessive force," she said.
She was asked if she remains optimistic about Somalia.
"In Africa, you have to be," she replied.