Kenyan security forces are continuing to go from room to room inside the upscale Nairobi mall, firing protectively before entering unknown territory, in a bid to secure the complex a day after a siege that killed scores ended.

The process of retrieving bodies from inside the mall still had not begun on Wednesday morning - possibly indicating that the situation was not yet considered secure - though a city morgue official said his workers were preparing to go into the building soon.

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Fears persisted that some of the attackers could still be alive and loose inside the rubble of the mall, a vast complex that had shops for retailers like Bose, Nike and Adidas, as well as banks, restaurants and a casino.

The group behind the four-day takeover of the mall has claimed that Kenyan government forces used chemical weapons in their assault on the building, then carried out "a demolition'' that buried 137 hostages.

In a series of Twitter posts from an account believed to be genuine, al-Shabab said that "having failed to defeat the mujahideen inside the mall, the Kenyan government disseminated chemical gases to end the siege".

They added "to cover their crime, the Kenyan government carried out a demolition to the building, burying evidence and all hostages under the rubble".

Government denial

Government spokesperson Manoah Esipisu immediately denied the claim, telling the AP news agency that no chemical weapons were used and that the official civilian death toll remains 61.

He insisted floors of the mall collapsed after a fire started by al-Shabab fighters caused structural weakness in a third floor parking lot, which then came down onto the second floor and brought it down onto the first or ground floor.

"Al-Shabab is known for wild allegations and there is absolutely no truth to what they're saying," he said. But officials said the death count would likely rise. Estimates varied between only a few bodies to dozens of bodies
possibly still inside the mall.

Meanwhile, a British man was arrested in Kenya following the attack, Britain's Foreign Office said.

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The agency said in a statement on Wednesday that British officials were ready to provide assistance to the man. Officials would not provide his name or details.

He is believed to be in his 30s. Britain's Daily Mail newspaper said he was arrested on Monday as he tried to board a flight from Nairobi to Turkey.

The newspaper said his face was bruised and he was acting suspiciously. 

Meanwhile, Kenya began three days of national mourning for the victims of the attack on Wednesday.

President Uhuru Kenyatta told the nation the night before the fighters had been defeated. The siege began midday on Saturday, when the gunmen marched into the building, firing from automatic weapons and throwing grenades.

There were fears that the death toll could jump dramatically as security forces searched for bodies inside the mall.

Al Jazeera's Andrew Simmons, reporting from Nairobi, said: "Some are predicting it could go well beyond the hundred mark, possibly even double the death toll," he said.

Source: Al Jazeera and agencies