A British national arrested in Nairobi is "not of significant interest" to the investigation into the mall siege that killed at least 72 people, Britain's top diplomat in Kenya said on Wednesday.
The Foreign Office in London confirmed the comment by British High Commissioner Christian Turner, which had appeared on media websites, but provided no further details.
According to the Daily Mail newspaper, a 35-year-old Briton of Somali origin was arrested at Nairobi's Jomo Kenyatta airport as he attempted to leave Kenya on a Turkish Airlines flight.
A Foreign Office spokesman declined to comment on the details contained in the report, saying only that the British authorities were aware of the arrest of a Briton in the Kenyan capital and stood ready to provide consular assistance.
A Kenyan anti-terrorism police unit source also said a British citizen of Somali origin was detained after missing his flight at Nairobi airport, and was now being questioned. He gave no more details.
The Daily Mail said the man had attracted attention at the airport because he had bruising to his face, was wearing dark glasses and was behaving suspiciously.
The newspaper quoted Kenyan officials as saying the man's British passport appeared to be genuine and it contained a Kenyan visa, although there was no stamp indicating when and how he had entered the country.
The newspaper also said the man said under questioning that his facial injuries happened during a recent visit to Somalia.
Bomb disposal experts and investigators were searching through the wreckage of the Westgate shopping mall on Wednesday after a four-day confrontation with Islamist militants.