The prime minister is cutting short a visit to Balmoral to chair an emergency meeting on the Kenyan terror attack, which left three Britons dead.
At least 69 people have been killed in total and more than 170 injured since the attack began at the Nairobi shopping centre on Saturday.
The Somali al-Shabab movement has said it was behind the attack.
A commander has denied reports that some of the attackers were of British or American nationalities.
Defence Secretary Philip Hammond is chairing a meeting of Cobra, the government's emergency response committee, on Monday morning. David Cameron will chair another meeting later.
It will be the fourth Cobra meeting, which is a get together of ministers, civil servants, the police, intelligence officers, on the Kenyan attack.
At least 170 people have been injured since the attack began on Saturday.
Between 10 and 15 attackers are still inside the Westgate shopping centre. Heavy gunfire and explosions have been heard on Monday morning.
The Foreign Office has said families of the British victims have been informed. It also expects the number of British fatalities to rise.
Mr Cameron, who has warned the UK to be braced for "more bad news", called the attack "an absolutely sickening and despicable attack of appalling brutality".
"These appalling terrorist attacks that take place where the perpetrators claim they do it in the name of a religion - they don't," he said.
"They do it in the name of terror, violence and extremism and their warped view of the world. They don't represent Islam or Muslims in Britain or anywhere else in the world."
On Monday, he tweeted: "I'm cutting short a visit to Balmoral to return to Downing St to chair COBRA late this afternoon, dealing with the Kenya terror attacks."
The UK has offered Kenya assistance, including intelligence co-operation.
UK staff from Addis Ababa in Ethiopia travelled to assist locally-based officials and a rapid deployment team was sent from London, the Foreign Office said.
Shoppers have told of how they huddled in back hallways as gunmen threw grenades and then opened fire.
Foreigners from France, Canada, India, the Netherlands, South Africa, Ghana and the US were also killed in the attack, including a Canadian diplomat working at Canada's High Commission to Kenya.
On the run
A Twitter account purporting to be an al-Shabab spokesman, which has now been shut down, named a 24-year-old man from London as one of the gunmen.
But an al-Shabab commander, who called himself Abu Omar, has denied reports that some of the attackers were of British or American nationalities.
He told the BBC such reports were "baseless" - as were reports that women had also stormed the shopping centre.
A later Twitter account, also claiming to be from al-Shabab, named Samantha Lewthwaite.
Media reports suggest Ms Lewthwaite, the widow of 7 July bomber Jermaine Lindsay, was involved in the attack but the Foreign Office says it has no evidence and is unwilling to speculate.
Ms Lewthwaite, who comes from Aylesbury, is on the run and is wanted by Kenyan police over links to a suspected terror cell.
Dubbed the "White Widow" by many of the papers, it is understood she fled across the border from Kenya to Somalia in 2012.
Al-Shabab says it carried out the attack in response to Kenyan military operations in Somalia.
The group, which is part of the al-Qaeda network, 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, where they have been fighting the militants since 2011.
British nationals concerned about friends or family can contact the Foreign Office on +44 (0)20 7008 0000.