Kenyan police said Saturday that between four and six men conducted a deadly Nairobi mall siege, far fewer than thought, as chilling new footage showed the attackers calmly ambling around with AK-47 rifles in hand.

Kenya initially said that between 10 and 15 gunmen were involved in the 80-hour siege of a busy upmarket shopping centre that killed at least 67 people.

"From what we have now that is coming out of the investigation, the number of attackers was between four to six," police chief David Kimaiyo told Kenyan television station KTN.

"None of them managed to escape from the building after the attack," he said, implying that they were killed in the confrontation.

Witnesses described how the fighters stormed the complex midday on September 21, when it was crowded with Saturday shoppers, firing from the hip and hurling grenades.

Newly released CCTV camera footage from one part of the mall shows four armed men walking calmly around, apparently searching for new victims.

Two weeks since Somalia's Al-Qaeda-linked Shebab insurgents attacked Nairobi's upmarket Westgate mall, the CCTV footage could help clear up some of the many unanswered questions about the bloodbath -- not least the attackers' identity.

On Saturday Kimaiyo confirmed that wanted British "White Widow" Samantha Lewthwaite -- reported to have been one of the attackers -- was not involved.

"On Samantha we have also established that she was not part of the attackers in the building. There was no woman," he said.

A police source said that at least three -- one Kenyan and two Somalis -- had been identified, but it remains unclear whether any of the attackers were Western.

Kenyan officials claimed to have killed five assailants, but the footage, taken around five hours into the 80-hour siege, shows only four men.

One thing is clear: the men were organised, apparently unafraid, and utterly ruthless.

The security camera footage shows them in the bread section of the supermarket hours after the siege began, the bulk of their victims most likely already dead.

They wear backpacks, perhaps stuffed with the ammunition that they would use to keep Kenya's army -- backed by foreign special forces -- at bay for four days.

In one shot, one of the attackers approaches supermarket checkout tills, knocking over computer screens.

At the same time, he is seen chatting on a mobile phone, suggesting either coordination with attackers inside the mall or even to commanders outside or in Somalia.

Fate of gunmen in footage unknown

The Shebab issued a running commentary throughout the attack via various Twitter accounts, claiming to be in contact with the fighters inside the mall.

It could provide key clues for detectives, not least because the supermarket area -- where the footage was shot -- was later engulfed in a fierce blaze.

Kenyan and foreign investigators are continuing to comb the burnt-out mall for clues to the perpetrators and their operating methods.

It is not clear what happened to those seen in the footage: whether they died in a shootout as Kenya claims, or in the fire that gutted part of the mall, or indeed if they managed to escape.

Part of the rooftop carpark collapsed on top of the supermarket, leaving the key site where the insurgents are understood to have made their final stand -- along with possible hostages -- buried under tonnes of concrete slabs and rubble.

Thirty-nine people are still listed as missing by the Red Cross.

The Shebab have warned they will step up militant attacks against Kenya after Nairobi refused to pull its troops out of Somalia, warning last week in the latest in string of threats that "rivers of blood will flow in Nairobi".

Kenya invaded southern Somalia to attack Shebab bases two years ago, and later joined the 17,700-strong African Union force deployed in the country.

Meanwhile foreign special forces staged a nighttime sea and air attack on a key Shebab base in southern Somalia, apparently targeting a senior Islamist commander. The Shebab said they fought off the assault.