The State Department urged US citizens to "exercise increased vigilance" if staying in hotels around Nairobi and to be aware of buildings' emergency evacuation plans.
In Kenya a US embassy alert warning of the threat went out to citizens, while police on Thursday released a "periodic security alert" saying security had been stepped up as there were "a few criminal elements who may wish to cause harm to the public."
"We wish to caution the public to be extra vigilant even as the police and other security agencies continue to work around the clock to ensure the safety and security of our country and Kenyans at large," read the statement, which gave no specific details.