Kenyan security forces are combing the Nairobi shopping centre attacked by suspected al-Shabab militants, as they seek to secure the site.
An explosion and bursts of gunfire were heard coming from the Westgate complex on Tuesday morning, as speculation grew that the operation was nearing its end.
At least 62 people have been killed and more than 170 injured.
Meanwhile, Kenya’s foreign minister said “two or three” Americans and a British woman were among the attackers.
In an interview with the US TV programme PBS Newshour, Amina Mohamed said the Americans were 18 or 19 years old, of Somali or Arab origin, and lived “in Minnesota and one other place”.
She said the Briton was a woman who had “done this many times before”.
Ms Mohamed appeared to contradict earlier comments from Kenya’s interior minister, who suggested that all the attackers were men – though some
may have been dressed as women.
The BBC’s Mike Wooldridge in Nairobi says Ms Mohamed’s remarks have fuelled media speculation about the possible involvement of Samantha Lewthwaite, the widow of one of the men who carried out attacks on London’s transport system on 7 July 2005.
The Somali Islamist al-Shabab movement said it carried out the attack in retaliation for Kenyan military operations in Somalia.
British officials said they would not be drawn on the identity of the attackers while investigations continue.
The Kenyan Red Cross has told the BBC that 51 people remain unaccounted for.
Early on Tuesday a Kenyan officer at the scene said he believed there could be two or three attackers left inside the building.
The interior ministry said it believed all hostages had been released – though messages on a Twitter feed purporting to speak for al-Shabab said hostages were still being held and militants were “holding their ground”.
The BBC understands from official sources that six of the attackers have been killed – three on Monday and three since midnight.
Officials said earlier that 10 people had been arrested in connection with the attack.
Kenyan Interior Minister Joseph Ole Lenku told the BBC late on Monday that the operation was in its final stages.
“The terrorists could be running and hiding in some stores, but all floors now are under our control,” he said. “There is no room for escape.”
Between 12 and 15 militants stormed the Westgate centre on Saturday, throwing grenades and firing on shoppers and staff.
At least 18 foreigners are 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.
President Barack Obama called the attack a “terrible outrage” and said the US was providing all the co-operation it could to Kenya.
Thousands of Kenyans have been responding to appeals for blood donations.
Al-Shabab, which is linked to al-Qaeda, has repeatedly threatened attacks on Kenyan soil if Nairobi did not pull its troops out of Somalia.
There are about 4,000 Kenyan troops in the south of Somalia as part of an African Union force supporting Somali government forces.
Al-Shabab is fighting to create an Islamic state in Somalia.
Despite being pushed out of key cities in the past two years, it remains in control of smaller towns and large swathes of the countryside.
UN special representative for Somalia Nicholas Kay called on Tuesday for a fresh surge in African troops to Somalia to counter an estimated 5,000 al-Shabab fighters, Reuters news agency reported.