The U.S. embassy in Uganda has warned American citizens in the country to stay indoors overnight Saturday because Ugandan authorities have warned of a terrorist plot.

The embassy in a message Saturday on its website said it is trying to assess the scope of a possible plot by the Somali militants of al-Shabab and whether members of the cell plotting the attack are still at large. It did not give details of steps being taken by Ugandan authorities or when or how the attack was to take place. The statement urged U.S. citizens to stay indoors to give authorities time to establish heightened security measures.

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