In a series of Twitter posts from al-Shabaab's official account, they said that "having failed to defeat the mujahideen inside the mall, the Kenyan govt disseminated chemical gases to end the siege."
They also added that "to cover their crime, the Kenyan govt carried out a demolition to the building, burying evidence and all hostages under the rubble."
Government spokesman Manoah Esipisu immediately denied the claim, telling The Associated Press that no chemical weapons were used.
"Al-Shabaab is known for wild allegations and there is absolutely no truth to what they're saying," he said.
The al-Shabaab extremists stormed the mall on Saturday, throwing grenades and firing on civilians.
The group used Twitter to say that Somalis have been suffering at the hands of Kenyan military operations in Kenya, and the mall attack was revenge.
"You could have avoided all this and lived your lives with relative safety," the group Tweeted on Tuesday. "Remove your forces from our country and peace will come."
The militants specifically targeted non-Muslims, and at least 18 foreigners were among the dead, including six Britons, as well as citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China.
The mall attack was the deadliest terrorist attack in Kenya since the 1998 al-Qaeda truck bombing of the U.S. Embassy in Nairobi, which killed more than 200 people.