Authorities fear the death toll from Kenya's terror attack could rise further as the nation begins three days of mourning.

Kenya has begun three days of mourning for the victims of Nairobi's shopping centre attack, as authorities prepare to retrieve the bodies of victims still trapped inside.

At least 67 people have been confirmed dead in the attack, including 61 civilians and six security officials.

Kenyan authorities say the number of dead may rise further, as bodies are feared to be trapped beneath rubble in the mall after three levels collapsed.

Six Britons - including an eight-year-old girl - are among the dead, and Defence Secretary Philip Hammond has warned there may be further British victims.

A Western embassy official said the number of additional dead could reach as high as 100. Morgue workers are preparing to receive further victims.

"They're just seeing dead bodies. They've found no survivors, no live hostages," a Nairobi resident, who did not wish to be named, told The Associated Press.

About 175 people were also wounded when al Shabaab militants stormed the Nairobi mall on Saturday afternoon.

Kenya's President Uhuru Kenyatta announced in a televised address that troops had "shamed and defeated" the militants following a four-day siege.

"Kenya has stared down evil and triumphed. These cowards will meet justice as will their accomplices and patrons, wherever they are," Mr Kenyatta said.

Some 11 suspects have been arrested in connection with the attack, and a further five died during the siege at the complex.

Police have confirmed that the attackers are either dead or in custody.

The Foreign Office has confirmed the arrest of a Briton, and say they are making contact to offer standard consular assistance.

"Now it is for the forensic and criminal experts," said police spokesman Masoud Mwinyi.

Explosives experts are searching for booby traps in the wreckage.

"They are checking for any potential explosive devices left behind," a security source told the AFP news agency, adding that specialist remote-controlled demining robots were on hand.

Teams of sniffer dogs have been taken into the building to check for explosives and to find bodies.

Workers wore face masks and some soldiers wrapped scarves around their mouths to cover what they said was an overpowering stench inside the centre.

It is believed that police have come across a large number of booby traps, and a large explosion was heard at the centre on Wednesday morning.

"Any suspicious objects they'll blow up," he said. "There's a security fear here still that maybe some of the gunmen are still alive."

The attackers marched into the four-storey, part Israeli-owned mall at midday on Saturday, shooting dead shoppers with machine guns and tossing grenades.

Al Shabaab has claimed it carried out the attack in retaliation for Kenya's military intervention in Somalia.

"The world is full of suffering. It is also full of overcoming"