In an emergency warning, the US mission in Kampala said it had "received information of possible terrorist threats to locations where Westerners, including US citizens, congregate in Kampala, and that an attack may take place soon".
"Out of an abundance of caution, the US mission has cancelled some non-essential events scheduled at local hotels in the coming days," the embassy warning said, telling US citizens they "should expect increased security sweeps and delays when entering or exiting hotel areas".
Uganda's police chief Kale Kayihura said the militants were planning strikes against "vulnerable places" in Kampala including hotels, shopping malls, markets and parks.
"I have no doubt that with the vigilance of the public and the time-tested effectiveness of our security forces we shall once again defeat even this latest, desperate attempt by evil forces," he said in a statement.
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Uganda is a key contributor to the African Union's AMISOM force battling the Shebab inside Somalia, and a key east African ally of the United States.
A Western diplomatic source also described the threat as "serious" and "based on concrete information".
The US embassy in Kampala last issued an emergency warning in September 2014. Ugandan and Western security sources later revealed they had intercepted a transfer of explosive suicide vests by suspected Shebab members.
US officials said at the time that the militants -- who have also been losing ground inside Somalia in the face of an AMISOM offensive -- were determined to avenge the killing of their leader Ahmed Abdi Godane in a US air strike earlier that month.