NAIROBI, Kenya — The militant group behind the takeover of a Nairobi mall claimed today the Kenyan government assault team carried out “a demolition” of the building, burying 137 hostages in rubble. A government spokesman denied the claim and said Kenyan forces were clearing all rooms, firing as they moved and encountering no one.

In a series of tweets from a Twitter account believed to be genuine, al-Shabab also said that “having failed to defeat the mujahideen inside the mall, the Kenyan govt disseminated chemical gases to end the siege.” It did not specify which gases, which could theoretically include anything from tear gas to poison.

Government spokesman Manoah Esipisu told The Associated Press that no chemical weapons were used, the collapse of floors in the mall was caused by a fire set by the terrorists and the official civilian death toll remains 61.

“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 will likely rise. Estimates varied between only a few bodies to dozens of bodies possibly still inside the mall.

In another development, a British man was arrested in Kenya following the terrorist attack, Britain’s Foreign Office said.

The agency said in a statement today British officials are 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 Monday as he tried to board a flight from Nairobi to Turkey with a bruised face and while acting suspiciously.

President Uhuru Kenyatta told the nation the night before the terrorists had been defeated and declared three days of national mourning beginning today.