The 63 recorded missing are thought to include both hostages still being held by Islamist militants fighting Kenyan troops inside the complex, as well as those possibly killed in the three-day-long siege.
Others may be hiding in the sprawling centre.
As the stand-off entered its third day, sustained bursts of rapid gunfire erupted at dawn and lasted 15 minutes, and soldiers posted around the mall ducked for cover.
Somali Shabaab militants on Monday threatened to kill hostages they are holding as Kenyan troops move to end their siege.
Kenyan Interior Minister Joseph Ole Lenku said in a security update Monday that “a few” hostages remain in the mall, but would not give precise figures.
The Shabaab have not said how many people were being held by the dozen-or-so attackers, who marched into the four-storey building at midday Saturday, spraying shoppers with machine gunfire and tossing grenades.