Some of the protesters on Thursday accused authorities of a variety of misdeeds, including killing or jailing members of the opposition.
State Department spokesman Richard Boucher said he had no overall assessment of the conduct of the elections but agreed that the charges needed to be looked into.
Both the ruling and opposition parties were claiming victory in the 15 May balloting.
The official results are expected to be announced on 8 June.
Boucher said the United States has made clear its view that the vote count and other aspects of the process "need to be done in a transparent and democratic manner."
Washington has a large Ethiopian community made up of people who fled the country in the mid-1970s after Emperor
Haile Selassie was overthrown and their descendents.