NAIROBI, Kenya -- Huge columns of black smoke poured out of the besieged mall in Nairobi on Monday afternoon as Kenyan forces moved in for a major assault against heavily armed Islamist militants who stormed the mall Saturday, slaughtering dozens of civilians and taking hostages.
Kenyan forces have been struggling to capture or kill the militants, who are holed up with military-grade weaponry and seem determined to fight to the death. The security forces' tactical operations inside the mall seemed to be intensifying on Monday, with long bursts of gunfire ringing out and several large explosions.
There were conflicting reports on civilian and military deaths and injuries, but it seemed clear that more than 60 people had been killed -- including three Britons -- and at least 150 injured, while at least a handful of hostages remained.
Western security advisers said that three Kenyan commandos had been killed on Sunday night while trying to dislodge the attackers, though Kenyan officials have reported only injuries to military forces.
The Shabab, a militant group in Somalia that has killed countless civilians with suicide bombs, has claimed responsibility for this attack and boasted of its prowess in a barrage of Twitter messages. The group has said that the attack is retribution for the Kenyan military presence in Somalia, where Kenyan troops have helped drive Shabab fighters out of much of the territory they once controlled.
But the assault was the deadliest terrorist attack in Kenya since the 1998 Qaeda truck bombing of the American Embassy in Nairobi, which killed more than 200 people. One Western security adviser was among those who said other terrorist groups might be involved. "This whole thing seems more advanced than anything the Shabab has ever done," he said.
A confidential United Nations security report described the assault on the mall as two-pronged, with groups of gunmen attacking on different floors simultaneously.
The security adviser said that the three Kenyan commandos were killed along with several hostages on Sunday night, as the military forces moved in against a group of militants hiding in a dark corner of the mall. The Kenyan soldiers were shot several times, at close range, and may have been executed while they were wounded and still alive, the adviser said.
"It was either a trap or an ambush," said the adviser, who asked not to be identified during the operations.
The security advisers said that 5 to 10 hostages were killed in the episode, either caught in the cross-fire or executed by the militants, and that three militants were killed. Kenyan officials said two militants were killed and several others wounded, and they have not commented on the hostages.
Kenya has called on Israeli military advisers to help them. Members of American, British and French security services were also on the scene, a high-ranking Kenyan official said Monday, though it seemed that the Israelis were the only outsiders involved in helping plan the combat operations.
Several witnesses said that some of the ringleaders of the attack, in which masked militants moved methodically through the mall, fatally shooting men, women and children, may have escaped during the initial confusion. Some said that one assailant, who did not look African, quickly tore off his clothes and changed into a new outfit and then ran out, blending in with a crowd of panicked, fleeing civilians.
Several security officials in Nairobi said that there may have been two women among the militants who also escaped after the initial stage of the attack, raising fears that well-trained terrorists could be on the loose in Nairobi. Some witnesses said that some of the militants were not African and may have been from Western countries, but American officials said they could not confirm that.
Already, Kenya's currency has begun to slide, raising fears that the attack will have devastating consequences on the economy, which is fueled by tourism and outside investment and is vulnerable to swings in perception.
The Kenyan authorities had vowed to end the assault Sunday night. On Monday, they acknowledged that the effort was taking longer than expected, but said that it would be over soon. "We are in charge of the situation, our people are safe, and hostages have been evacuated," said Joseph Ole Lenku, Kenya's interior cabinet secretary. Mr. Ole Lenku said that 10 Kenyan soldiers had been injured, and that he was "not aware" if any had been killed.
All day Monday, police helicopters swirled over the mall and a surveillance aircraft pulled low, growling circles. "We are also closing in on the attackers," David Kimaiyo, the inspector general of the National Police Service, said on Twitter in the afternoon.
Col. Cyrus Oguna of the Kenya Defense Forces said the number of hostages "cannot be more than 10 now."
"Most areas of the building are now in the control of K.D.F.," he told the television station KTN.
The Shabab warned on Twitter that "Kenyan forces who've just attempted a roof landing must know that they are jeopardizing the lives of all the hostages at #Westgate." Another Web posting, quoted by Reuters and ascribed to the militants, issued a threat to kill the remaining hostages if force was used. The newspaper The Standard reported that the militants were protected in a bulletproof room.
On Monday, reflecting the breadth of the crisis, judges at the International Criminal Court in The Hague took the unusual step of suspending for one week the trial of Kenya's deputy president, William Ruto, on charges of crimes against humanity so that he could return home to assist in the government's response.
Mr. Ruto has pleaded not guilty to three charges related to the violence that rocked the country after the disputed 2007 election.
President Obama called the Kenyan president, Uhuru Kenyatta, on Sunday to reaffirm the "strong and historic partnership between the United States and Kenya." That relationship has been strained by the election in March of Mr. Kenyatta, who is being prosecuted at the International Criminal Court on charges of financing death squads during the 2007 political violence. Mr. Obama skipped visiting Kenya, his father's birthplace, on his trip to Africa in late June.
Addressing the nation, Mr. Kenyatta sounded a note of solidarity in loss, revealing that his nephew and the man's fiancée were among the dead. "These are young, lovely people I personally knew and loved," Mr. Kenyatta said. "Many of us have lost loved ones. Let us mourn them all as one nation and keep them always in remembrance and prayer."
He said security forces had rescued more than 1,000 people from the mall since the violence began on Saturday, calling the forces' performance "remarkable and encouraging," even as he asked for patience from the public as the standoff continued.
The mall is one of Nairobi's fanciest, and the expensive shops and restaurants attracted wealthy Kenyans and expatriates alike. Among the dead were people from all over the world, including from Britain, France, Ghana, Canada, China and India. After the confirmation of the deaths of the three Britons, Prime Minister David Cameron said Monday that he was cutting short a visit to Scotland and returning to London to head a meeting of the so-called Cobra committee of senior ministers and security officials. Five Americans were among the wounded, but none were known to have been killed. News agencies reported that other foreigners were also among the dead.
The killings have plunged Kenya into national mourning.
Among the dead was Ruhila Adatia-Sood, a popular Kenyan radio host who was in the parking lot of the mall hosting a cooking competition, according to reports. She posted several photographs on her Instagram account before the attack.
Also killed was Kofi Awoonor, 78, a Ghanaian poet and a former professor at the University of Ghana.
Kenya's political class, often starkly divided, has demonstrated unity since the attack. Raila Odinga, a former prime minister and a political opponent of Mr. Kenyatta, joined him in an appearance at the State House on Sunday.
"This is a trying moment for our country," Mr. Odinga said. "It is something that has hit at the very heart of our country's unity. Our people must come together at times like this to help each other."
In his address on Sunday, Mr. Kenyatta, too, appealed for unity. "We have ashamed and defeated our attackers," he said. "Let us continue to wage a relentless moral war as our forces conduct a physical battle. We shall triumph."
Reporting was contributed by Reuben Kyama and Tyler Hicks from Nairobi; Jodi Rudoren from Jerusalem; Mark Mazzetti from Washington; Mohammed Ibrahim from Mogadishu, Somalia; William K. Rashbaum from New York; and Alan Cowell from London.
This article originally appeared in The New York Times.