The general secretary of Nato has paid tribute to the "critical job" British forces are doing in Afghanistan as he warned failure there would give al Qaida's terrorist ambitions a "free run".
Jaap De Hoop Scheffer said the military alliance cannot afford to walk away from the country however dangerous or expensive the campaign becomes.
He acknowledged that it had been a "tragic period" for the UK and said the troops were part of a Nato team making a "shared sacrifice".
Speaking to the Chatham House foreign affairs think-tank, Mr De Hoop Scheffer - who stands down next week after five years at the helm of Nato - said: "Fourteen nations are fighting in the south of Afghanistan, alongside their British colleagues, along with Afghan forces. Hundreds of Nato soldiers from other countries have also lost their lives - which is a sad, but real, measure of shared sacrifice."
He added: "If we were to walk away, Afghanistan would fall to the Taliban, with devastating effect for the people there - women in particular," he said.
"Pakistan would suffer the consequences, with all that that implies for international security. Central Asia would see extremism spread. Al Qaida would have a free run again, and their terrorist ambitions are global.
"This is not conjecture. This is fact. Those who argue otherwise - who say we can defend against terrorism from home - are simply burying their heads in the sand."
He also issued a stark warning that Nato could not succeed in addressing the security challenges of the 21st century with a "20th century mindset" based on the old certainties of the Cold War.
While during the Cold War, the main purpose of the alliance's military forces was deterrence and not actually to be used, in the current environment forces had to be used, he argued.
"You cannot deter civil wars or suicide terrorists, nor can you deter states from collapsing," Mr De Hoop Scheffer said.
