The U.S. Embassy in Uganda warned American citizens visiting the country to stay home Saturday amid a potential terrorist threat. Ugandan authorities uncovered a previously unknown terrorist cell with ties to Somali militant group al-Shabaab in Kampala.

"Today Ugandan authorities reported the discovery of an al-Shabaab terrorist cell in Kampala, Uganda. We remain in close contact with our Ugandan counterparts as investigations continue into what appears to have been planning for an imminent attack,” embassy officials said in a statement, according to CBS News. “We urge that you exercise all possible caution, remaining at home or in a safe location until the all clear is issued. 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 statement gave no indication of when, where or how the possible attack would take place. Al-Shabaab vowed revenge against the United States after the death of leader Ahmed Abdi Godane Friday in a U.S. airstrike.

Ugandan police officials said Saturday they had uncovered and prevented a terrorist attack in Central Kampala, Reuters reported. Explosives were seized and an unspecified number of suspected militants were arrested in the raid, but a police spokesman would not say if the individuals were affiliated with al-Shabaab.

Local authorities are “increasing vigilance of security at all public places, particularly vulnerable places such as hotels, shopping places, bars and restaurants,” a police spokesman told the Wall Street Journal.

In 2013, al-Shabaab claimed responsibility for an attack that killed 67 people at Westgate Mall in Nairobi, Kenya. The group, which has ties to al Qaeda, was also responsible for the 2010 bombing of sports bars in Uganda during the World Cup.