Uganda's government spokesman, Ofwono Opondo, has accused the US of trying to "subvert elections", Reuters news agency reports.
US ambassador to Uganda Natalie Brown was on Monday stopped from visiting opposition leader, Robert Kyagulanyi, popularly known as Bobi Wine, at his home in the capital, Kampala.
"What she has been trying to do blatantly is to meddle in Uganda's internal politics, particularly elections, to subvert our elections and the will of the people," Mr Opondo told Reuters.
The US State Department had on Monday called for "independent, credible, impartial, and thorough investigations" into "many credible reports" of irregularities in Uganda's general election.
President Yoweri Museveni, who won a sixth term, has denied claims of election irregularities.