Sporadic gunfire and explosions were heard Monday from the shopping mall in Kenya's capital where Somali terrorists remained holed-up with an unknown number of hostages.
The standoff began around midday on Saturday, when an estimated 10 to 15 militants from the al Qaeda-linked group al-Shabab stormed the mall, shooting indiscriminately and hurling grenades inside the large shopping center popular with Westerners. At least 62 people have been killed in the attack, according to Kenya's Interior Minister Joseph Ole Lenku.
The Kenya Red Cross gave a slightly higher death toll earlier Monday, but tweeted out a correction later in the day. The Red Cross said another 175 people were injured and 63 people were still listed as missing. The U.S. State Department has confirmed that five Americans were among the injured.
Interior Minister Lenku said "almost all" of the hostages had been evacuated, but he did not say how many civilians or hostage-takers were still believed to be inside the mall.
After hours of sporadic clashes, a CBS News team positioned near the mall reported a heavy exchange of gunfire, several loud explosions and then thick black smoke emerging from within the complex. Lenku, the Interior Minister, said at his news conference that the smoke was due to the militants burning mattresses inside the mall.
With the mall surrounded by Kenyan police, soldiers and special forces, a spokesman for al-Shabab released an audio statement online warning that the remaining hostages would be killed if the security forces attempted to end the standoff by force.
CBS News correspondent Charlie D'Agata reported that, according to witnesses, the terrorists searched the mall over the weekend and asked anyone they encountered whether they were Muslim. The Muslims were largely allowed to leave the mall.
Police inspector general David Kimaiyo said Monday on his Twitter feed that security forces were "closing in" on the militants in the mall, according to the Reuters news agency, but he did not go into detail on the operations. He said several more hostages had been rescued.
"Israelis and Kenyan forces have tried to enter Westgate (mall) by force but they could not, the mujahideen (fighters) will kill the hostages if the enemies use force," a man identifying himself as Sheikh Ali Mohamud Rage said in the clip, according to the Reuters news agency. CBS News could not independently verify the authenticity of the audio statement, but it was posted on a website often used by the Somali terror group.
Westgate Mall is at least partially owned by Israelis, and reports circulated that the Kenyan forces were getting advice on the ground from both Israeli and U.S. military officials. There was no indication that U.S. forces were involved in the military operation.
A senior Israeli defense official said there were no Israeli forces participating in an assault, but the official said it was possible that Israeli advisers were providing assistance.
A Kenyan military spokesman said Monday the fate of hostages inside the upscale mall was not clear, despite earlier claims that most hostages had been rescued. Many civilians were seen leaving the mall over the course of Saturday, having escaped unaided, but it was unclear how many remained inside.
CBS News' D'Agata arrived Monday morning to Nairobi and said the city was tense. Even staff at the hotel in the capital city were taking additional precautions, searching bags and scrutinizing visitors more carefully than normal. Security forces had established a roughly 200 yard perimeter around the Westgate mall. Helicopters hovered overhead.
Kenyan police said late Sunday on Twitter that a "MAJOR" assault by security forces was ongoing to end the siege. "This will end tonight. Our forces will prevail. Kenyans are standing firm against aggression, and we will win," Kenya's Disaster Operations Centre said on Twitter.
Kenya Defence Forces later said it had rescued most hostages and had taken control of most of the mall. Officials did not immediately release the number of hostages rescued or the number that remained. Four Kenyan military personnel were wounded in the operation, the military said.
It is not the first time terror has struck in Kenya. In 2002, militants bombed an Israeli-owned luxury hotel near Mombasa, killing 13 people, and tried to shoot down an Israeli airliner at the same time. Kenyans and foreigners were among those confirmed dead, including British, French, Canadians, Indians, a Ghanaian, a South African and a Chinese woman.
Somalia's al-Shabab, claimed responsibility for the attack specifically targeting non-Muslims, saying it was retribution for Kenyan forces' 2011 push into neighboring Somalia.
Speaking by phone to the BBC on Monday, another al-Shabab spokesman identifying himself as Abu Omar denied reports that U.S. nationals and women were among the militants involved in the attack on the mall. Al-Shabab has drawn a significant number of young Somali-American men from the U.S. as recruits, but there was no confirmation from U.S. officials as of Monday morning regarding involvement in the Westgate attack by U.S. nationals.
While known to have links to al Qaeda, al-Shabab has for years been focused on carrying out attacks primarily inside Somalia. There have been suggestions of divisions among the organization's leadership causing tension, and it has not typically been considered the greatest threat to U.S. interests -- certainly not in comparison to the Yemen-based al Qaeda franchise al Qaeda in the Arabian Peninsula (AQAP).
Last week, however, new information surfaced in the form of a document filed in a court case in New York's Eastern District, alleging that al-Shabab is or was operating a "research and development department" aimed at producing chemical weapons.
U.S. Secretary of State John Kerry, meanwhile, condemned over the weekend what he called "an enormous offense against everybody's sense of right and wrong," and called the attackers "ruthless and completely reckless terrorists."
Kerry, who was in New York for meetings at the United Nations, spoke Sunday with Somalia's foreign minister and U.N. ambassador.
Kenyan President Uhuru Kenyatta reiterated his government's determination to continue fighting al-Shabab.
"We went as a nation into Somalia to help stabilize the country and most importantly to fight terror that had been unleashed on Kenya and the world," said Kenyatta."We shall not relent on the war on terror."
He said although the violent attack had succeeded, Kenyan security forces had "neutralized" many others.
Earlier in the day Kenyatta said he his nephew and his nephew's fiance were killed in the attack.
Kenyan security officials sought to reassure the families of hostages inside but implied that hostages could be killed. The security operation is "delicate" because Kenyan forces hoped to ensure the hostages are evacuated safely, said Interior Cabinet Secretary Joseph Lenku.
"The priority is to save as many lives as possible," Lenku said, adding that more than 1,000 people escaped the attack inside the mall on Saturday.
"We have received a lot of messages from friendly countries, but for now it remains our operation," Lenku said.
CBS News' travel editor Peter Greenberg, who is visiting Nairobi, described the Westgate as "a very large mall, with more than 80 stores and large grocery stores and banks," that is popular with locals and tourists alike.
At least four Britons were confirmed dead in the attack on Monday morning, after Britain's prime minister warned his nation on Sunday to "prepare ourselves for further bad news" when the U.K. death toll stood at three. There are approximately 40,000 British nationals living in Nairobi.
In addition to the Britons, Kenyans, French, Canadians, Indians, a Ghanaian, a South African and a Chinese woman had been confirmed dead.
The United Nations Security Council condemned the attacks and "expressed their solidarity with the people and Government of Kenya" in a statement.