As Kenya began three days of mourning Wednesday for at least 67 civilians and security personnel killed in the siege of a Nairobi mall, it was unclear how many hostages may have died with the Somali attackers buried in the rubble. Three floors in a part of the mall had collapsed near the end of the operation, leaving an unknown number of bodies under steel and concrete. It was unclear what caused the structure to come down. Kenyan officials and Somali armed group Al-Shabab made contradictory statements Tuesday as to who had the upper hand in the weekend attack . In a televised address late Tuesday, President Uhuru Kenyatta said that Kenyan forces had "ashamed and defeated" Al-Shabab at the Westgate mall, though he did not explicitly state that the operation had been completed. "As a nation our head is bloodied but unbowed," Kenyatta said, adding that his forces killed five fighters and took 11 into custody. However, in a tweet believed to be from Al-Shabab, the group said 137 hostages were killed and blamed the Kenyan government for the deaths. "Kenyan govt employed chemical agents to end the siege at #Westgate Mall, killing all the hostages in the process," the tweet said. Al-Shabab also maintained that its fighters were holding out in the mall and said that hostages they had seized were still alive. "There are countless number of dead bodies still scattered inside the mall, and the Mujahideen (fighters) are still holding their ground #Westgate," the group said on its Twitter feed. Kenyan police had earlier said in a tweet, "Troops now in mop-up operations in the building." The police urged people to ignore "enemy ... propaganda" and assured that the defense forces were continuing to "neutralize" the threat. Al-Shabab says it launched the attack in pursuit of demands that Nairobi withdraw troops from Somalia, where Kenya has battled the armed group. Kenyatta has vowed to stay the course there.

Foreign fighters?

Images from closed-circuit television inside the mall during the attack, published in a Kenyan newspaper on Tuesday, showed two fighters casually dressed and wearing ammunition belts. One held an assault rifle.



Via its Twitter account, Al-Shabab confirmed that the two men were part of the group that attacked Westgate.



Kenyan Foreign Minister Amina Mohamed told "PBS NewsHour" that "two or three Americans" and a British woman were among the militants.



She said the Americans were "young men, about between maybe 18 and 19" years old. She said they were of Somali or Arab origin and had lived in "in Minnesota and one other place."



Al-Shabab, which said it was in communication with its members in the mall, dismissed the minister's comments.



"Those who describe the attackers as Americans and British are people who do not know what is going on in Westgate building," Al-Shabab's media office told Reuters.



The FBI told Al Jazeera that, as of Tuesday afternoon, it could not confirm reports that Americans were among the Al-Shabab fighters.



President Barack Obama, whose father was born in the East African nation, offered U.S. help, saying he believed Kenya -- the location of one of Al-Qaeda's first major attacks, in 1998, and a neighbor of chaotic Somalia -- would continue to be a regional pillar of stability.



Kenyan officials have tried to reassure the country that they are in command of the situation.



"We continue to appeal for calm, keep vigil and avoid Westgate area," the Ministry of Interior said on its Twitter account.



The attack on the mall is the worst such incident in Kenya since Al-Qaeda killed more than 200 people when it bombed the U.S. Embassy in Nairobi in 1998.



Kenya has sent troops to Somalia as part of an African Union force trying to push back Al-Shabab and stabilize the country, which was long without a functioning government.



Kenya has also suffered internal instability. Kenyatta, who lost a nephew in the weekend bloodbath, faces charges of crimes against humanity at the International Criminal Court for his alleged role in coordinating violence after disputed elections in 2007. He denies the charges.



British Defense Secretary Philip Hammond said he believed six Britons had died in the attack. Other known foreign victims are from China, Ghana, France, the Netherlands and Canada. Kenyan officials said the total death toll was at least 67.



Kenyatta declared three days of mourning in honor of the Westgate Mall victims.



Al Jazeera and Reuters