While the government announced on Sunday that "most" hostages had been released, a security expert with contacts inside the mall said at least 10 were still being held by a band of attackers described as "a multinational collection from all over the world".
"Despite botched attempts by the Jews and Christians to recapture the mall today, the mujahideen are still in control of the Westgate," al-Shabab spokesman Sheik Ali Mohamud Rage said in the message. "The upper hand still remains theirs," he said.
Kenyan Foreign Minister Amina Mohamed said "two or three Americans" and "one Brit" were among those who attacked the mall.