"President Obama called President Kenyatta of Kenya this morning to express condolences to the government and people of Kenya for the terrorist attack carried out by al-Shabaab yesterday on the Westgate Shopping Mall in Nairobi," the White House said in a statement.
"President Obama reiterated US support for Kenya's efforts to bring the perpetrators of the attack to justice."
A State Department official said there were no new reports of American casualties in the attack.
Republican Representative Peter King of New York, a member of the House Intelligence Committee, said the attack showed the growing influence of al Qaeda linked groups in Africa.
"This shows the really growing influence of al Qaeda in Africa. You have al Shabaab now in Somalia going off into Kenya, carrying out an attack outside its own country," King said on ABC's "This Week."
The attack showed that "al Qaeda and its affiliates are still extremely powerful and still able to really strike terror into the hearts of people, attacking a shopping mall, it has no military significance at all," King said.