It said the cell belonged to Somali Islamist group al-Shabab, but this has yet to be confirmed by Ugandan police.
The embassy earlier warned US citizens in the capital to stay at homeduring a police operation to uncover the cell.
Ugandan police say they have increased security in public places across Kampala, and made several arrests.
Earlier this week, the US embassy warned of possible revenge attacks against US targets by al-Shabab in response to the US air strike that killed the group's leader Ahmed Abdi Godane on 2 September.
'Stay at home'
"We're increasing patrols in the city, major towns and other vulnerable places," police spokesman Fred Enanga told reporters on Saturday.
In addition to making several arrests, the police said they had seized explosive materials.
The suspects are believed to be foreigners, says the BBC 's Catherine Byaruhanga.
Soldiers and police were out in force in a similar alert in July following a threat to Uganda's main airportThe US embassy urged US citizens "to stay at home or proceed to a safe location" while the Ugandan authorities were conducting operations against a suspected al-Shabab terrorist cell in Kampala, in a message on its website.
It said it was not aware of specific targets but that security had been increased at key sites, including Entebbe international airport.
Al-Shabab has vowed to retaliate for the killing of Godane.
The Islamist group, which wants to overthrow the UN-backed government in Somalia, has since named Ahmad Umar as the new leader.
The group was behind twin blasts targeting a rugby ground and Ethiopian restaurant in Kampala that killed 76 football fans watching the World Cup final in July 2010.
At the time, the then-leader Ahmed Abdi Godane said the attack was retribution for Uganda's deployment of troops as part of the African Union joint force to help Somalia combat al-Shabab militants.
Source: BBC
