Somalia's al-Shabaab insurgents have claimed 137 hostages they seized died in the Nairobi shopping mall siege, although there is no confirmation of this.
The figures, from an insurgent Twitter post, are impossible to verify and are significantly higher than the number of people officially registered as missing, the news agency AFP reports.
Separately, Britain's Foreign Office says a British national has been arrested in Nairobi over the attack.
The BBC reports it comes after reports a man was arrested acting suspiciously at Jomo Kenyatta airport.
Kenyan President Uhuru Kenyatta announced an end to the 80-hour bloodbath late on Tuesday with what he called the immense loss of 61 civilians and six members of the security forces. Police say the death toll is provisional, with the Red Cross listing 63 people as still missing.
Kenyans have begun three days of official mourning, with flags flying at half mast, while rescue workers continue to scour the wreckage of the Westgate mall for bodies.
Officials say several bodies - including those of attackers - are thought to be trapped under rubble after three floors of the building collapsed. Explosive experts are also searching for possible booby-traps in the wreckage.
The insurgent Twitter post also accused Kenyan troops of using "chemical agents" to end the four-day stand-off.
"In an act of sheer cowardice, beleaguered Kenyan forces deliberately fired projectiles containing chemical agents," one tweet read.
"To cover their crime, the Kenyan government carried out a demolition to the building, burying evidence and all hostages under the rubble."
There was no immediate response from Kenya's government, but al-Shabaab have in past made repeated outlandish claims, especially on their Twitter site, AFP says.
The al-Qaeda linked group says it carried out the attack in retaliation for Kenya's two-year battle against the extremists' bases in Somalia.
Close to 200 people were wounded in the four-day siege, which saw running battles between militants and security forces in the complex, Nairobi's largest shopping centre and popular with wealthy Kenyans, diplomats, United Nations workers and other expatriates.