Several large blasts have rocked Kenya's Westgate Mall where a hostage siege is in its third day.
Associated Press reporters on the scene Monday heard several blasts and a barrage of gunfire.
Black and grey smoke rose up from the mall.
Security forces have been attempting to rescue an unknown number of hostages inside the mall held by al-Qaida-linked terrorists.
Four Americans are reported to be injured from the terrorist attack on a shopping mall in Kenya.
U.S. officials say the wife of a foreign service national working for the U.S. Agency for International Development was killed.
Somalia's Islamic extremist group al-Shabab claimed responsibility.
No details about the injured Americans were released by the State Department, cited privacy issues.
Secretary of State John Kerry called the attack "a heartbreaking reminder that there exists unspeakable evil in our world which can destroy life in a senseless instant."
"Attacks like this can't change who we are, a people committed to peace and justice for all, but rather must reaffirm our determination to counter extremism and promote tolerance everywhere," Kerr said in a statement.
Noting the death of the wife of the USAID worker, Kerry commended agency workers for their efforts around the world and said the U.S. pledged "our commitment to do whatever we can to assist in bringing the perpetrators of this abhorrent violence to justice, and to continue our efforts to improve the lives of people across the globe."
In a statement from the White House, a spokeswoman for the National Security Council said the U.S. would help bring the perpetrators of the attack to justice.
"We will continue to stand with the Kenyan people in their efforts to confront terrorism in all its forms, including the threat posed by al-Shabaab," NSC spokeswoman Caitlin Hayden said.
"This cowardly act against innocent civilians will not shake our resolve."
Fifty-nine people are dead and 179 people have been wounded from this attack on Saturday.