The U.S. Embassy in Nairobi suspended all U.S government temporary personnel, and told Mission personnel to shelter in place, avoid the Westgate mall area and avoid demonstrations or gatherings as the situation, according to its Facebook page.
Harf said U.S. officials were closely monitoring the situation.
"We condemn this senseless act of violence that has resulted in death and injury for many innocent men, women, and children. Our condolences go out to the families and friends of all victims. We have reports of American citizens injured in the attack, and the U.S. Embassy is actively reaching out to provide assistance," Harf said in a statement.
ABC News' Jeffrey Kofman contributed to this report.