Kenyan and foreign forensics teams scoured the wreckage of a Nairobi shopping mall Wednesday for bodies and clues after a four-day siege by Islamist gunmen left 67 dead and dozens more missing.

Rescuers and investigators wore face masks and some soldiers wrapped scarves around their mouths because of an overpowering stench inside the Westgate centre, once the capital's most upmarket mall. A large part of the complex has collapsed after heavy explosions and a fierce fire.

Across Kenya, flags flew at half mast at the start of three days of official mourning.

Somalia's Shebab chief Ahmed Abdi Godane said the Nairobi mall carnage was a "message to Westerners" who had "backed Kenya's invasion (of Somalia) that has spilled the blood of the Muslims for the interest of their oil companies".

In an audio message posted on an Islamist website, Godane threatened "more bloodshed" unless Kenya withdrew its troops from Somalia.

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

The Al-Qaeda-linked group claimed on Twitter that 137 hostages they had seized all died, figures impossible to verify and higher than the number of people officially registered as missing.

It also accused Kenyan troops of using "chemical agents" and explosives to end the stand-off.

President Uhuru Kenyatta announced an end to the 80-hour bloodbath late Tuesday, with the "immense" loss of 61 civilians and six members of the security forces. Five suspected attackers were also killed, and 11 detained, officials said.

Police said the death toll was provisional, with the Kenyan Red Cross reporting 71 people missing.

Interior Minister Joseph Ole Lenku said top forensic experts and investigators from Britain, the United States, Israel, Germany, Canada and Interpol had joined the probe.

But he was unable to answer many remaining questions over the identity of the attackers, the possible presence of a British woman and American jihadists, and how the cell got such large quantities of weapons and ammunition into the complex.

"It is an elaborate process. Among the things that are going on now are fingerprinting, DNA identification (and) ballistic examinations," Lenku said.

An AFP reporter saw teams with sniffer dogs entering the bullet-riddled mall, apparently to check for explosives and victims buried under the rubble of a collapsed part of the building. Forensic teams could take at least a week to gather evidence, Lenku said.

"The army told us we would get access to the bodies yesterday, but then said it was too dangerous," a Kenyan Red Cross official told AFP.

"The bodies that are still inside the mall will have to be identified from photos. They are now in such a state of decomposition that you can't put a family member through that."

In one of the worst attacks in Kenya's history, the militants marched into the four-storey, part Israeli-owned mall at midday Saturday, spraying shoppers with automatic weapons fire and tossing grenades.

"There was blood everywhere," one soldier who entered the mall during the siege said. "Some bodies were burnt and others rotting."

The attack was meticulously planned and prepared, with fighters stocked with enough ammunition to hold off Kenyan forces backed by American, British and Israeli agents. According to US officials cited by the New York Times Wednesday, the attackers had blueprints of the mall and had hidden powerful belt-fed machine guns in one of the stores beforehand, perhaps with the help of a corrupt local employee.

The siege saw running battles between militants and security forces in one of Nairobi's largest and most modern shopping centers. The mall is popular with wealthy Kenyans, diplomats, UN workers and other expatriates, and was packed when the attack began.

Close to 240 were wounded in the attack, the health ministry said.

The siege developed into a hostage drama with the Islamists claiming to hold many civilians and gunmen running and hiding in supermarket aisles, store rooms, a cinema and casino and placing booby traps.

As well as scores of Kenyans -- from ordinary workers to the president's nephew -- many of the dead were foreigners, including from Britain, Canada, China, France, the Netherlands, India, South Africa and South Korea.

Five attackers were also killed and 11 suspects detained, Kenyatta said, vowing "full accountability for the mindless destruction, deaths, pain, loss and suffering we have all undergone."

"These cowards will meet justice, as will their accomplices and patrons, wherever they are," the president vowed, saying investigations were under way to "establish the nationalities of all those involved".

There has been growing media speculation over the possible role of wanted British extremist Samantha Lewthwaite, daughter of a British soldier and widow of suicide bomber Germaine Lindsay, who blew himself up on a London Underground train on July 7, 2005, killing 26 people.

Families of those still missing are anxiously waiting for news of their relatives, with the Red Cross and expert counselors and psychologists setting up tents at Nairobi's morgue to offer support to grieving relatives. (AFP)