An explosion and gunfire have been heard at the shopping centre in Nairobi where militants are believed to be holding a number of hostages.
Sixty-eight people have been killed and more than 170 injured since the attack began on Saturday.
Between 10 and 15 attackers – thought to be militants from the Somali al-Shabab movement – are still inside the Westgate shopping centre.
Reporters at the scene said there had been heavy and rapid bursts of fire.
An unnamed Kenyan security source told the AFP news agency that an army assault was underway.
An AFP correspondent at the scene said he heard about 15 minutes of fierce gunfire which then subsided.
A photographer accompanying the correspondent said troops deployed around the mall were forced to duck for cover.
The photographer said it “sounded as if the shots were coming from somewhere around the mall, or were being fired from a vantage point in the mall”.
The defence forces said on Twitter several hours ago: “All efforts are under way to bring this matter to a speedy conclusion.”
Some civilians are still trapped, either as hostages or because they are hiding.
Overnight reports said that the gunmen were holed up in a supermarket.
Earlier defence spokesman Col Cyrus Oguna said only a small number of hostages were still being held and most had been rescued.
As troops continue to clear the building it was possible they would come across more bodies, Col Aguna warned on Sunday.
Correspondents at the complex reported an eerie calm early on Monday despite the army’s statements that it wanted to bring a “speedy conclusion” to the siege, now entering its third day.