NAIROBI, Kenya — The U.S. embassy in Uganda warned American citizens in the country Saturday to stay at home overnight as Ugandan authorities attempt to foil a possible terror attack.

The embassy in a message on its website said it is trying to assess the scope of a possible plot by the Somali militants, al-Shabab, and whether members of the cell plotting the attack are still at large.

The warning did not have details of when or how the attack was to take place. The statement urged U.S. citizens to remain at home to give authorities time to establish heightened security measures.

“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. 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 said.

Al-Shabab, an Islamic extremist group linked to Al-Qaida, has vowed to revenge the killing of its former leader last week by a U.S. airstrike.

The Islamic extremists from Somalia four years ago bombed two World Cup viewing locations in neighboring Kampala, Uganda, killing more than 70 people. One year ago al-Shabab claimed responsibility for the attack on Westgate Mall in Nairobi, Kenya, that killed 67 people. Ahmed Abdi Godane, who was the spiritual leader of al-Shabab killed by the U.S. airstrike, said both attacks on Kenya and Uganda were retribution for the countries troop deployment to Somalia to fight the militants.

Kenya, Uganda, Ethiopia, Djibouti and Sierra Leone have deployed troops to Somalia as part of the African Union joint force to bolster the country’s weak government from an al-Shabab insurgency.

In a week, Kenya will commemorate the anniversary of the Westgate Mall attack. Authorities in Kenya say they are on high alert for possible attacks by al-Shabab as the anniversary approaches.

Al-Shabab vowed to avenge Godane’s death while appointing little known Ahmad Umar as their new leader a week ago. The Somali capital was struck a day after the threat by mortars. A few days later a suicide car bomber targeting an African Union troop convoy in the Lower Shabelle region killed 12 civilians.