The United States on Friday criticised police actions in the run-up to Uganda's national polls next month.
"Excessive force, obstruction and dispersal of opposition rallies, and intimidation and arrest of journalists have contributed to an electoral climate of fear and intimidation, and raise questions about the fairness of the process," the State Department said.
Uganda is one of Washington's closest allies in Africa and a recipient of large sums of US development and military aid.
While the US spoke out strongly two years ago against an anti-homosexuality proposal in Uganda's Parliament, the Obama administration has refrained from criticising President Yoweri Museveni's decision to seek to extend his 30 years in power.
The US has recently condemned moves by other African leaders to remain in office beyond constitutionally prescribed term limits, which do not apply in Uganda's case.
"Free and fair elections depend on all Ugandans being able to exercise their right to assemble peacefully, express their opinions, and participate in the electoral process free from intimidation and abuse," the State Department declared on Friday.
"They also depend on government institutions and security forces remaining neutral, defending the rights of all people and protecting all parties equally."
The statement by department spokesman John Kirby expressed "particular concern" over the reported disappearance of opposition campaign aide Christopher Aine.
Washington also expressed criticisms on Friday concerning human rights abuses in Ethiopia, another country that serves as a strategic US ally in East Africa.
"The United States is increasingly concerned by the continued stifling of independent voices in Ethiopia, including the detention of Oromo political party leaders," the State Department said.
"These arrests have a chilling effect on much needed public consultations to resolve legitimate political grievances in Oromia."
Human Rights Watch reported last week that Ethiopian security forces killed 140 people in Oromia State during recent protests over seizure of farmers' lands. The government has confirmed fewer than 10 deaths in the demonstrations that began in November.
The State Department said the US supports Ethiopian authorities' stated commitment to public consultations with affected communities.
But, the statement added, "for these consultations to be meaningful, all interested parties must be able to express their views freely."
The US also called for the release of all Ethiopians imprisoned "for exercising their rights, such as political party leaders and journalists."