Loud explosions sent thick smoke billowing from the Nairobi mall where militants from Somalia's al Qaida-linked al Shabaab group threatened to kill hostages on the third day of a raid in which at least 59 have already died.
The blast was followed by other smaller detonations and volleys of gunfire but there was no indication of the fate of people whom the authorities say are being held by 10 to 15 Islamist gunmen who took the complex by storm on Saturday. Sky News reported "major fighting" in the area.
As troops ran into new positions, security official told Reuters at the scene: "It is us who caused the explosion, we are trying to get in through the roof."
A plume of oily black smoke was still pouring from the building after 15 minutes.
A senior officer had said police were "closing in" on the militants after rescuing more trapped people overnight. But officials have been suggesting since Sunday that the siege may be near an end, while the guerrillas seemingly remain at large.
Interior Minister Joseph Ole Lenku said "a few" people were still hostage: "Our disciplined forces ... have been extremely careful to find the balance between neutralising the attackers and getting as many people to safety as possible," he added.
A spokesman for al Shabaab, which has demanded Kenya pull its troops out of neighbouring Somalia, warned they would kill hostages if Kenyan security forces, who are being assisted by Western and Israeli experts, tried to storm their positions:
"Israelis and Kenyan forces have tried to enter Westgate by force but they could not," Sheikh Ali Mohamud Rage said in an audio statement posted online. "The mujahideen will kill the hostages if the enemies use force."
On Twitter, the group posted: "As the operation gathers momentum inside #Westgate, the Mujahideen are for the 3rd day still in full control of the situation on the ground."
The Red Cross said on Monday that the death toll since Saturday's initial lunchtime bloodbath stood at 69. But Interior Minister Lenku said that was wrong and that only 59 were dead.