Westgate mall
© Boniface Muthoni/CorbisSmoke rises from the Westgate mall on Monday.
Amina Mohamed says Briton who has 'done this many times before' was involved in attack with 'two or three' Americans

A British woman who has allegedly taken part in terrorist activity "many times before" was among the attackers who laid siege to a Kenyan shopping mall, the country's foreign minister has said.

Amina Mohamed said the militant acted alongside "two or three" Americans during the atrocity in Nairobi which has killed at least 62 people, including six Britons.

The announcement will fuel speculation that the British terrorism suspect Samantha Lewthwaite, who was married to the 7/7 bomber Jermaine Lindsay, was involved.

Lewthwaite 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.

The Kenyan authorities had claimed all the militants involved in the three-day mall siege were men, but Mohamed appeared to contradict this in a US television interview on Monday.

"From the information that we have, two or three Americans [were involved] and I think so far I have heard of one Brit ... a woman ... and I think she has done this many times before," she told the PBS NewsHour programme.

Mohamed said the Americans were 18 to 19 years old, of Somali or Arab origin and lived "in Minnesota and one other place" in the US.

US officials said they were looking into whether any Americans were involved. State Department spokeswoman Jen Psaki said on Monday the department had "no definitive evidence of the nationalities or the identities" of the attackers.

White House spokesman Ben Rhodes said US officials have seen "reports coming out of al-Shabaab that indicate information along those lines", referring to possible involvement of Americans in the attack.

"But we have to run those to ground, of course," he said. "We do monitor very carefully and have for some time been concerned about efforts by al-Shabaab to recruit Americans or US persons to come to Somalia.

"This is an issue that has been tracked very closely by the US government, and it's one that we'll be looking into in the days ahead."