Kenya's Foreign Minister Amina Mohamed has said the woman had "done this many times before" and "two or three" Americans were also among the attackers.
Six British nationals are confirmed dead in the attack on the Westgate mall in Nairobi and there are conflicting reports over whether the siege has been ended yet.
The UK Foreign Office (FCO) said the next of kin of the six had been informed and were being offered consular support.
Lewthwaite, dubbed the 'White Widow', is known to be in East Africa and is wanted by Kenyan police over alleged links to a terrorist cell that planned to bomb the country's coast.
In March last year, officials said she had fled to Somalia and that officers were hunting a woman who used several identities, including hers.
Responsibility for the attack has been claimed by Al Shabab - an Islamist group based in neighbouring Somalia, which is part of the Al Qaeda network.
Meanwhile, Kenyan troops continued to battle the extremists in the deadly shopping mall siege today - the fourth day of the attack.
"We continue to liaise very closely with the Kenyan authorities and to support their investigation into this attack," and FCO spokesperson said.
Britain's security service MI5 is reportedly trawling records of Britons with links to extremists in Somalia amid concerns that sympathisers could follow up the shopping mall massacre in Kenya with terror attacks in the UK.
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
