"We'd like to see election monitors come in, we'd like to see UN human rights monitors come in and ensure we have a safe electoral process in Zimbabwe," White House spokesman Gordon Johndroe said in Texas.
"Opposition leaders and supporters must be able to freely campaign free of violence," he said.
Tsvangirai said earlier he would return home to compete with Mugabe in the runoff vote. He has set deploying election monitors as a precondition for contesting in the runoff.
Tsvangirai beat Mugabe, who has ruled the southern African nation since independence in 1980, in the March 29 election in Zimbabwe but not by enough votes to avoid a runoff.
