Kenyan Foreign Minister Amina Mohamed, interviewed by US public broadcaster PBS, was asked about reports Americans and Britons were among those to blame for the carnage.
"Yes," Mohamed said. "From the information that we have, two or three Americans, and I think so far I have heard of one Brit." The Briton was a woman, Mohamed said.
"Woman. Woman. And she has, I think, done this many times before," she told PBS.
"The Americans, from the information we have, are young men, about between maybe 18 and 19," she said, adding they were of Somali or Arab origin but "lived in the US, in Minnesota and one other place."
"So, basically, look, that just was to underline, I think, the global nature of this war that we're fighting," Mohamed added.
Meanwhile, the Kenyan interior ministry announced that troops were "in control" of Nairobi's Westgate shopping mall, with all the hostages trapped by Islamist gunmen believed to have been freed.
A government spokesman said the three-day-long siege, in which the attackers massacred at least 62 shoppers and staff, was close to being declared over. He said special forces combing the building were no longer encountering any resistance.
Somalia's al Qaeda-linked al Shabab insurgents have claimed the attack, which began Saturday, when the gunmen marched into the complex, firing grenades and automatic weapons and sending panicked shoppers scrambling for their lives.
"I think we're all shocked," Mohamed said on the sidelines of a United Nations event in New York.
"And what does it tell us It tells us that, as governments, we must do better," she said. "If they can cooperate at that level, if they coordinate their evil at that level, that governments around the world must cooperate even more ... to just make sure that we stay ahead of the curve.
"This is a totally new way of doing business for them. And I think we have just seen how much damage can be done," Mohamed said.