Loud explosions and heavy gunfire have been heard at the Westgate shopping centre in Nairobi, hours after Kenyan forces launched a major attack against al-Shabaab militants who are holding hostages inside the mall.

Onlookers at the scene of the attack said an army-led operation against the heavily-armed militants is likely to get underway, in what appears to be a final assault against the Islamists to bring the siege to an end.

At least three explosions have been heard during the latest stand-off.

This is said to be the biggest operation against the attackers since they took the mall under their control on 21 September.

Kenya's Defence Forces had earlier said they will bring the siege "to a speedy conclusion".

An AFP photographer outside the mall 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 sustained gunfire during the morning attack went on for nearly 15 minutes and later subsided.

The al-Qaida-affiliated insurgents continue to hold hostages inside the shopping centre. However Kenyan security forces said most of the hostages have been rescued.

It is still unclear whether the civilians still inside the mall are being held as hostages or they are hiding from the attackers.

The siege has so far officially claimed 68 people and injured more than 170.

As the crisis enters the third day, Kenya's Disaster Operation Centre earlier said: "This will end tonight. Our forces will prevail. Kenyans are standing firm against aggression, and we will win."

Ahead of the latest drive to bring the siege to an end, the Kenya Defence Forces wrote on their Twitter account: "All efforts are under way to bring this matter to a speedy conclusion."

A newly-created Twitter account for al-Shabaab had warned the military against their ongoing operation.

"Kenyan government shall be held responsible for any loss of life as a result of such an imprudent move. The call is yours!" said the Somali militant outfit.

"Kenyan forces who've just attempted a roof landing must know that they are jeopardising the lives of hostages."