Kenyan foreign minister Amina Mohamed told US public broadcaster PBS that two or three American men and a British woman were among those to blame for the carnage.
"And she has, I think, done this many times before," she told PBS.
A spokesman for the foreign office in London said the ministry was "aware" of Mohamed's comment.
"We continue to liaise very closely with the Kenyan authorities and to support their investigation into this attack," he said.
"The UK will do everything it can to support the Kenyans bringing everyone responsible for this barbaric attack to justice."
Somalia's al Qaeda linked 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.
At least 62 shoppers and staff have been killed and close to 200 wounded in the siege.