However, he noted that the US embassy remained open and consular services were functioning. Washington has been "in close contact" with Kampala about anti-government riots last week that Duguid said left 21 dead and 100 wounded. The Ugandan government puts the death toll at 14.
"We urge the government of Uganda to ensure that the rights of its citizens are respected under Ugandan and international law," said Duguid.
"American citizens living in Uganda have been urged to exercise extreme caution and to avoid those areas affected by the unrest."
He also urged all Americans to register with the US embassy in order to receive messages from the consular service as soon as possible.
Ugandan President Yoweri Museveni meanwhile vowed tough action against protesters following riots in the capital city Kampala last week.
Around 600 people were arrested and more than 200 of them charged Monday in connection with the protest against the government's move to block the traditional king from visiting a rural district outside Kampala.
Ronal Muwenda Mutebi, the king of the Baganda -- Uganda's most populous ethnic group -- was to visit the county northeast of Kampala last weekend, but the trip was disallowed for fears it would stoke violence.
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