It said it had "received information of possible threats" at city locations where Western nationals gather.
Uganda's government said the US had warned of a possible suicide bomber trying to enter the country.
Uganda was under threat because its troops were fighting militant Islamists in Somalia, a spokesman added.
In 2010, Somalia's al-Shabab militants carried out a suicide bombing in Kampala, killing 76 people as they watched the football World Cup final.
The US embassy said an attack "may take place soon".
Some planned events had been cancelled at Kampala hotels, it added.
Border controls have been tightened to stop the suspected suicide bomber and his accomplices from crossing into Uganda, government spokesman Ofwono Opondo told the BBC.
Uganda is a key contributor to the African Union mission fighting al-Shabab inside Somalia.
Last September, Uganda was stripped of the right to host an international cricket tournament because of security concerns.
The US embassy in Uganda has warned that Westerners - including Americans - may be targeted for "terrorist" attacks in the capital Kampala.