The U.S. Embassy in Kenya said Monday the timing of the attack is not known but they believe it to be in its final planning stages. They did not give further details.
The embassy urged Americans to be vigilant.
Al-Qaida-linked al-Shabab militants from neighboring Somalia have vowed to carry out an attack on Kenya for sending troops into Somalia.
Kenya sent hundreds of troops into Somalia in October last year following a wave of kidnappings, including those of four Europeans on Kenyan soil, blamed on al-Shabab.
