Kenyan authorities fear that the death toll following the siege at an upscale mall in the capital Nairobi would rise dramatically after it was discovered that three floors of the shopping complex had collapsed.

At least sixty-one civilians, five fighters and six members of the security forces are known to have died in the four-day siege which the Somali based armed group al-Shabab has claimed responsibility for.

But Al Jazeera's Andrew Simmons, reporting from Nairobi on Wednesday, said three floors of the building had collapsed during the siege and that the death toll was likely to be much higher.

"Some are predicting it could go well beyond the hundred mark, possibly even double the death toll," he said.

Kenya's president Uhuru Kenyatta announced late on Tuesday that the siege of Nairobi's Westgate shopping mall had ended, calling for three days of national mourning for the victims.

Kenyatta said that 67 people had been killed in the attacks by al-Qaeda linked gunmen and that 11 suspects were being held in custody.

"We have been badly hurt, but we have been brave, united and strong. Kenya has stared down evil and triumphed. We have defeated our enemies and showed the whole world what we can accomplish," he said in a televised address to the nation.

Explosive experts searched on Wednesday morning for possible booby-traps in the wreckage of the mall.

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

An AFP reporter outside the bullet-riddled mall also saw teams of sniffer dogs, which will check not only for explosives, but for the grim expectation of the bodies.

Kenyatta said "intelligence reports had suggested that a British woman and two or three American citizens may have been involved in the attack."

"We cannot confirm the details at present but forensic experts are working to ascertain the nationalities of the terrorists."

Somalia's al-Shabab rebels had dismissed speculation in a Twitter post on Tuesday that any woman was involved in an attack by its fighters.

"We have an adequate number of young men who are fully committed and we do not employ our sisters in such military operations," the group said.

Al-Shabab 'not acting alone'

In an interview on Monday, Kenya's Foreign Minister Amina Mohamed told Al Jazeera that al-Qaeda, not al-Shabab, was behind the attack.

Al-Shabab is "not acting alone", this assault is "part of an international terrorism campaign", Mohamed said.

She said that about 20 gunmen and women were behind the attack, and that both the victims and perpetrators came from a variety of nationalities.

"Al-Shabab are looking for relevance on an international scale - especially after a change of leadership - and is looking to send the message that they are still a force to be reckoned with," Al Jazeera's Mohamed Adow said.

In a speech on Monday, the Somali president, Hassan Sheikh Mohamud, described al-Shabab as a threat to the world.

"They are a threat to the continent of Africa, and the world at large," he said.

Al-Shabab and al-Qaeda announced their alliance in February last year, and Abu Omar, an al-Shabab commander, confirmed in an interview with Al Jazeera that his group is taking orders from al-Qaeda.

The siege began midday on Saturday, when the gunmen marched into the complex, firing grenades and automatic weapons and sending panicked shoppers fleeing.

The dead include six Britons, two French women, two Canadians including a diplomat, a Chinese woman, two Indians, a South Korean, a South African and a Dutch woman.

Also killed was Ghanaian poet and former UN envoy Kofi Awoonor, 78, while his son was injured.