The US State Department has called for "independent, credible, impartial, and thorough investigations" into "many credible reports" of irregularities in Uganda's general election last week.
Long-time leader Yoweri Museveni was re-elected for a sixth term amid accusations of vote fraud by his main rival Bobi Wine.
The president said the poll could be the "most cheating-free" in the history of the African nation.
In a statement, the US State Department said those found culpable of election irregularities should be held accountable.
Its spokesman Morgan Ortagus also condemned "the continuing attacks on political candidates" and urged the government to "respect their human rights and fundamental freedoms, including the freedom of expression".
Opposition politicians Bobi Wine has told the BBC that he was not being allowed to leave his house which was surrounded by security forces. On Sunday he said his family had run out of food.