Ugandan authorities have foiled a planned imminent attack by the Somali militant Islamist terror group al-Shabab, the U.S. Embassy in Uganda said Saturday.
"We are continuing our engagement with Ugandan authorities as we seek to assess the scope of the disrupted al-Shabab terrorist plot and whether there are members of the cell still at large."
The embassy urged all U.S. citizens to continue to "shelter in place" through the evening hours.
"At this point we are not aware of specific targets, and the Ugandan authorities have increased security at key sites, including Entebbe International Airport," the embassy said.
Police spokesman Fred Enanga confirmed Ugandan forces had "made arrests" but gave no details of how many people had been detained.
It was not immediately possible to determine the target of the reported attack.
Al-Qaida-linked al-Shabab claimed responsibility for the attack on the Westgate shopping mall in Nairobi in neighboring Kenya a year ago, in which 67 people died.
The group has vowed revenge for the killing of its leader in a U.S. airstrike in Somalia earlier this month.
"We also caution U.S. citizens of the possibility of retaliatory attacks in Uganda by al-Shabab in response to the U.S. and Ugandan military actions in Somalia last week which killed al-Shabab leader Ahmed Godane."
As one of the countries that contribute forces to an African Union peacekeeping mission battling the Islamist militant group in Somalia, Uganda has suffered militant attacks in recent years, and al-Shabab has threatened more.
In 2010, al-Shabab bombed sports bars in Uganda where people were watching the soccer World Cup on television.