Three British nationals have been confirmed dead after the terror attack in Nairobi, with the Foreign Office warning the number could rise.
Prime Minister David Cameron described the assault on the Westgate shopping centre as an "absolutely sickening and despicable act of appalling brutality". Mr Cameron said people in the UK should prepare for "further bad news" as the situation developed.
UK staff from Addis Ababa in Ethopia have travelled to assist with the efforts in Nairobi and a rapid deployment team had been sent from London, the Foreign Office said. Foreign Secretary William Hague has chaired a meeting of the Government's emergency committee Cobra to co-ordinate the response to the situation.
A sombre Mr Cameron said: "It is an absolutely sickening and despicable attack of appalling brutality. What we know is that three British nationals have been killed."
He added: "Because the situation is ongoing we should prepare ourselves for further bad news. Our thoughts also should be with the Kenyan government and the Kenyan people at this time." Mr Cameron said he had offered the Kenyan authorities "every assistance" in a call to president Uhuru Kenyatta.
Mr Cameron said: "These appalling terrorist attacks that take place, where the perpetrators claim they do it in the name of a religion, they don't. 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.
"Obviously the first priority is to make sure we do everything for the British nationals caught up in this, so we're strengthening the consular services that we have and the people that we have to look after our people. We've also offered the Kenyans help in terms of policing and advice and intelligence collaboration and other areas which we will be able to help with."
A day after 10 to 15 gunmen stormed the Westgate Mall attacking shoppers with guns and grenades, an unknown number of hostages remained in the building. Somali-based militant group al-Shabaab claimed responsibility for the outrage at the mall in the affluent Westlands district of the capital, which is popular with expats.
Government officials said at least 59 people were confirmed dead and 175 others were wounded in the atrocity, while about 1,000 were rescued from the upmarket mall. President Kenyatta has pledged to hunt down and punish the terrorists. In a televised address he said he had "personally lost family members in the Westgate attack".
The Labour Party conference in Brighton paused for a minute's silence as news of the deaths filtered through, with Ed Miliband among those reflecting on the tragedy. Concerned British nationals are advised to monitor Foreign & Commonwealth Office (FCO) travel advice and to contact 020 7008 0000.