Updated at 4:10 a.m. Eastern
NAIROBI, Kenya Kenyan security forces battled al Qaeda-linked terrorists in an upscale mall for a fourth day Tuesday in what they said was a final push to rescue the last few hostages in a siege that has left at least 62 people dead.
Despite Kenyan government assurances of success on Twitter, another explosion and more gunfire could be heard coming from the mall at around 6:30 a.m. local time Tuesday. Security forces carried a body out of the mall, which remained on fire, with flames and smoke visible.
"Our forces are combing the mall floor by floor looking for anyone left behind. We believe all hostages have been released," the Reuters news agency quoted the Ministry of Interior as saying on Twitter early Tuesday. He said government forces were "in control" of the building.
Al-Shabab, however, directly challenged that message in its own Twitter post, claiming the militants in the mall were "still holding their ground."
The terror group also challenged the Kenyan government's claim that all hostages had been rescued, saying the captured civilians were, "still alive, looking quite disconcerted but, nevertheless, alive."
Kenyan officials insisted in briefings Monday that there were few, if any hostages left inside, reports CBS News correspondent Charlie D'Agata. They said it remained unclear how many militants were holed up in the building.
While the government announced Sunday that "most" hostages had been released, a security expert with contacts inside the mall told The Associated Press at least 10 were still being held by a band of attackers described as "a multinational collection from all over the world."
Kenyan Foreign Minister Amina Mohamed saidwere among those who attacked the mall.
She said in an interview with the PBS "NewsHour" program that the Americans were 18 to 19 years old, of Somali or Arab origin and lived "in Minnesota and one other place" in the U.S. The attacker from Britain was a woman who has "done this many times before," Mohamed said.
U.S. officials said they were looking into whether any Americans were involved. State Department spokeswoman Jen Psaki said Monday that the department had "no definitive evidence of the nationalities or the identities" of the attackers.
The security expert, who insisted on anonymity to talk freely about the situation, said many hostages had been freed or escaped in the previous 24-36 hours, including some who were in hiding.
However, there were at least 30 hostages when the assault by al-Shabab militants began Saturday, he said, and "it's clear" that Kenyan security officials "haven't cleared the building fully."
Somalia's al-Qaeda-linked rebel group, al-Shabab, which claimed responsibility for the attack, said the hostage-takers were well-armed and ready to take on the Kenyan forces.
Flames and dark plumes of smoke rose Monday above the Westgate shopping complex for more than an hour after four large explosions rocked the surrounding neighborhood. The smoke was pouring through a large skylight inside the mall's main department and grocery store, where mattresses and other flammable goods appeared to have been set on fire, a person with knowledge of the rescue operation told The Associated Press.
The explosions were followed by volleys of gunfire as police helicopters and a military jet circled overhead, giving the neighborhood the feel of a war zone.
By Monday evening, Kenyan security officials claimed the upper hand.
"Taken control of all the floors. We're not here to feed the attackers with pastries but to finish and punish them," Police Inspector General David Kimaiyo said on Twitter.
Kenya's Interior Minister Joseph Ole Lenku said the evacuation of hostages had gone "very, very well" and that Kenyan officials were "very certain" that few if any hostages were left in the building.
But with the mall cordoned off and under heavy security it was not possible to independently verify the assertions. Similar claims of a quick resolution were made by Kenyan officials on Sunday and the siege continued. Authorities have also not provided any details on how many hostages were freed or how many still remain captive.
Three attackers were killed in the fighting Monday, Kenyan authorities said, and more than 10 suspects arrested. Eleven Kenyan soldiers were wounded in the running gun battles.
An al-Shabab spokesman, Sheik Ali Mohamud Rage, said in an audio file posted on a militant website that the attackers had been ordered to "take punitive action against the hostages" if force was used to try to rescue them.
The attackers have lots of ammunition, the militant group said in a Twitter feed, adding that Kenya's government would be responsible for any loss of hostages' lives.
A Western security official in Nairobi who insisted on not being named to share information about the rescue operation said the only reason the siege hadn't yet ended would be because hostages were still inside.
Westgate mall, a vast complex with multiple banks that have secure vaults and bulletproof glass partitions, as well as a casino, is difficult to take, the official said. "They are not made for storming," he said of the labyrinth of shops, restaurants and offices. "They're made to be unstormable."
At least 62 people were killed in the assault Saturday by some 12 to 15 al-Shabab militants wielding grenades and firing on civilians inside the mall, which includes shops for such retail giants as Nike, Adidas and Bose and is popular with foreigners and wealthy Kenyans.
The militants specifically targeted non-Muslims, and at least 18 foreigners were among the dead, including six Britons, as well as citizens from France, Canada, the Netherlands, Australia, Peru, India, Ghana, South Africa and China. Nearly 200 people were wounded, including five Americans.
Fighters from an array of nations participated in the assault, according to Kenya's Chief of Defense forces Gen. Julius Karangi. "We have an idea who these people are and they are clearly a multinational collection from all over the world," he said.
Al-Shabab, whose name means "The Youth" in Arabic, said the mall attack was in retribution for Kenyan forces' 2011 push into neighboring Somalia. It was the deadliest terrorist attack in Kenya since the 1998 al-Qaeda truck bombing of the U.S. Embassy in Nairobi, which killed more than 200 people.
An extremist Islamic terrorist force that grew out of the anarchy that crippled Somalia after warlords ousted a longtime dictator in 1991, al-Shabab is estimated to have several thousand fighters, including a few hundred foreigners, among them militants from the Middle East with experience in the Iraq and Afghanistan conflicts. Others are young,.
For years, Minnesota has been the center of a federal investigation into the recruiting of fighters for al-Shabab. Authorities say about two dozen young men have left Minnesota since 2007 to join the group. Minnesota's Somali community is the largest in the U.S.
Somali President Hassan Sheikh Mohamud said the attack showed that al-Shabab was a threat not just to Somalia but to the international community.
Mohamed, the Kenyan foreign minister, said her country needs to work with other governments to fight the increasing terrorist threat and "much more with the U.S and the U.K., because both the victims and the perpetrators came from Kenya, the United Kingdom and the United States.
"That just goes to underline the global nature of this war that we are fighting," she said.
Reports that some of the attackers may have been Somalis who lived in the United States illustrate the global nature of the militant group, the Somali leader said in a speech at Ohio State University. "Today, there are clear evidences that Shabab is not a threat to Somalia and Somali people only," he said. "They are a threat to the continent of Africa, and the world at large."
As the crisis passed the 48-hour mark, a video emerged that was taken by someone inside the mall's main department store when the assault began. It showed frightened and unsure shoppers crouching as long, loud volleys of gunfire could be heard.
Kenyans in many parts of the country stood in long lines Monday to donate blood to aid the nearly 200 people injured in the attack. Fundraisers raised hundreds of thousands of dollars, though government officials warned of scam artists taking advantage of the tragedy.