Gordon Brown has defended Britain's military mission in Afghanistan, saying: "We cannot, must not and will not walk away". In a keynote speech in London, hastily arranged following a series of British deaths in Afghanistan this week, Mr Brown said the military mission in the country was "our first line of defence" against terror attacks in the UK. Paying tribute to the 93 British troops who have died in Afghanistan this year, the Prime Minister said: "These men are our heroes today." Mr Brown added: "The main terrorist threat facing Britain emanates from Afghanistan and Pakistan. "Although the sustained pressure in Pakistan, combined with military action in Afghanistan, is having a suppressive effect on al-Qaeda, we know that they continue to train and plot attacks on Britain from the region "This mission must not fail. It is not easy; the choices are not simple. There is no strategy that is without danger and risk. But that is the responsibility of leadership - of Government, and of our armed forces. To do what is necessary, however difficult, to keep the British people safe."