KAMPALA, Uganda — The Ugandan authorities have uncovered a “terrorist cell” run by the Somali militant Islamist group the Shabab, which they said they believed was planning an imminent attack, the United States Embassy in Uganda said Saturday.

“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 in a statement on its website.

Uganda, as one of the countries that contribute forces to an African Union peacekeeping mission battling the Shabab’s fighters in Somalia, has been the scene of militant attacks in recent years, and the Shabab have threatened more.

The United States Embassy, shortly before issuing its announcement, told American citizens in Uganda, in a message posted on Twitter, to stay at home or proceed to a safe location while the Ugandan authorities completed operations against a suspected cell in Kampala, the capital.