British forces are likely to be among the last international forces to leave Afghanistan, Liam Fox has said.
The defence secretary said Helmand province, where the bulk of British troops are stationed, was one of the most "difficult parts" of the country.
As a result "the likelihood is that will be one of the last parts to transition over to Afghan security charge", he told the BBC.
The international coalition fighting in Afghanistan suffered its worst month in the country in June, when 102 soldiers lost their lives.
Dr Fox's visit to America this week has seen him drum up international solidarity as growing numbers of commentators warn the counter-insurgency strategy, which sees soldiers placed among the people rather than in secure positions, is not working.
"We have to show we're willing to see through this particular time, difficult though it is. We require strategic patience," the defence secretary urged.
"The upside of the counterinsurgency is when we show our commitment and convince the Afghan people we're on their side... that's when we get better intelligence and ultimately that's what provides a more secure environment."
The Taliban issued a defiant statement following controversy over plans to negotiate with disillusioned elements of the group.
"We do not want to talk to anyone - not to [president Hamid] Karzai, nor to any foreigners - till the foreign forces withdraw from Afghanistan," a spokesperson told the BBC.
"We are certain that we are winning. Why should we talk if we have the upper hand, and the foreign troops are considering withdrawal, and there are differences in the ranks of our enemies?"
In America, as in Britain, politicians face an increasingly difficult task persuading the public that the mission in Afghanistan is essential to national security.
Dr Fox's characteristically blunt assessment seeks to make the link between terrorism in Afghanistan and the terrorist threat in Britain even clearer.
He warned: "We have to ensure that when we leave we do not leave behind a security vacuum into which the forces of terror can be drawn again."