Kenya's president has confirmed he received intelligence reports suggesting a British women and a number of Americans were involved in the attack on a shopping mall in Nairobi.
"We cannot confirm the details at present but forensic experts are working to ascertain the nationalities of the terrorists," he said in a televised address.
Earlier comments by Kenya's foreign minister that a Briton and two or three Americans took part in the stand-off fuelled speculation that British terror suspect Samantha Lewthwaite, who was married to the July 7 bomber Jermaine Lindsay, was involved.
Asked about the British woman, Amina Mohamed told the US programme PBS Newshour: "A woman, and I think she has done this many times before."
However, Mrs Mohammed comments have been dismissed by an al Shabaab spokesman who said: "Those who describe the attackers as Americans and British are people who do not know what is going on."
Ms 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 Ms Lewthwaite, from Aylesbury, Buckinghamshire, had fled to Somalia and that officers were hunting a woman who used several identities, including hers.
On Monday night, Ms Lewthwaite's grandmother, Elizabeth Allen, 86, of County Down, was said by a friend to be "deeply distressed" by suggestions her granddaughter had been involved in the attack.
Councillor Raj Khan, whose family knew Ms Lewthwaite's family socially in Aylesbury, said he was surprised at speculation she was involved in the attack. He called her an "average, British, young, ordinary girl".
Ms Lewthwaite has gained "semi-mythical status" since travelling to East Africa, according to terrorism expert Raffaello Pantucci, a senior research fellow at the Royal United Services Institute.
Mr Pantucci said: "I don't think we've had any concrete evidence of her being involved in this incident, but the fact of her being mentioned in this context is not surprising because of her connections, and it is known that she is somewhere in East Africa."
Professor David Anderson, an expert in African terrorism at the University of Warwick, said Ms Mohamed's remarks must be taken with a "pinch of salt".
He said: "It would be very unusual if a female, and a British female at that, was involved in an al Shabaab operation."
Britain's Foreign Office would not confirm claims about the involvement of a British woman, while US officials said there was no evidence of the nationalities or identities of the attackers.
Speaking in Pakistan, Home Secretary Theresa May told the BBC she would not be commenting on reports that a British woman was involved.
According to The Times, FBI sources are investigating claims that the terrorists were recruited in a Somali community known as "Little Mogadishu" in Minneapolis, Minnesota.
Somalia's al Qaeda-linked al Shabaab group has claimed responsibility for the attack, which began on Saturday and has left 67 people dead.