Earlier Mr Ainsworth told the BBC: "We have to have patience; we've got to get behind our people who are doing the job on our behalf.
"It's not going to be sorted out within the next few weeks or the next few months - Afghanistan has been wrecked."
But during his speech at Chatham House Mr Ainsworth also insisted that there was now a "momentum" behind the international military campaign.
He said that a major British and US offensive currently under way in Helmand was succeeding in isolating the Taliban in its southern stronghold.
Rising death toll
And the defence secretary said there were "compelling reasons" for the UK's commitment to the operation.
"For Britain to be secure, Afghanistan needs to be secure," he said.
Mr Ainsworth said the focus was now on preventing a return to Taliban control in Afghanistan while preparing the way for presidential elections later this year.
He said that parallel to these efforts the international coalition was working to provide the time and space for Afghan forces to take on responsibility for the security of their own people, and for the Afghan government to build up their civil society.
The British death toll in Helmand Province is rising, prompting some to question the direction of the campaign.
UK forces are on a major offensive against the Taliban ahead of presidential elections in Afghanistan.
American troops are also fighting in the south of Helmand, with the death rate rising among US troops as well as British forces.
Seven American soldiers died on Tuesday and in the past week seven British soldiers also died.
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