The defence secretary said he could not put a time limit on Britain's commitment, but wanted to see "significantly more progress in the next year or so".
He said the government was aiming to speed up the training of Afghan soldiers, from 2,000 a month to 4,000, which would allow the Afghan army to reach its target strength by November 2010, ahead of schedule.
Foreign Secretary David Miliband later told the conference that the way to defeat the Taliban was to "separate the hard core from the rest".
To do this, he said, meant talking to the Taliban, to say: "Live within the constitution and you can come home to your communities and have a share of power, but stay outside, in hiding, linked to al-Qaeda, plotting mayhem for Afghanistan, and you will face unremitting military force."
Mr Miliband added that at present, ordinary Afghans "don't know who is going to win, and so don't dare give us all the backing we need".
Changing this would depend on making those people feeling safer by having more Afghan troops to protect them, he said.
He added: "We also know that a successful plan depends on a government in Kabul acting in the interests of the country, not in lining the pockets of the people close to power.
"So, we will wait to get a credible election result, and we will not be rushed into a whitewash."
The UK currently has about 9,000 military personnel in Afghanistan - the second largest deployment of any nation - and Mr Ainsworth said he would only consider increasing that if "other nations step up to the plate".
Any boost in numbers would also only happen if the government was "absolutely certain" they would have the right equipment.
This is not a head-on fight with the Taliban
"Yes, we want to be part of a very important coalition... but we also have to be mindful of our ability to supply our own troops," he told the BBC Radio 4's Today programme.
"The kit and equipment they need to do this job doesn't come from Marks and Spencer. It has to be ordered through a very complex procurement process."
Boosting troop numbers above 10,000 could stretch armed forces capacity beyond manageable levels, meaning people would be "going to theatre far more often than is appropriate or good for them", the minister added.
During his speech, Mr Ainsworth also accused the Conservatives of "ignoring the facts" about the supply of equipment and vehicles to personnel.
"False claims don't just damage the government; they risk damaging the morale of both our troops and the public. They risk damaging the mission," he said.
Gordon Brown told BBC Political Editor Nick Robinson that the case for being in Afghanistan was "as strong as it was in 2001".
"This is not a head-on fight with the Taliban," he said. "We've got to get the public in Afghanistan to recognise it's their country and we're helping them take back more control of their own affairs.
"Both Barack Obama and I are looking at how we can train up the Afghan army, train up the Afghan police, improve the civil institutions... and it's at that point that I believe we can reduce the troop numbers in Afghanistan."
Mr Brown has also denied that the Sun's switch to backing the Tories had derailed his plans to launch a Labour fight-back at the conference.
The newspaper cited Afghanistan - and in particular alleged shortages of vital equipment - as a key reason for abandoning Labour.
But despite this, the PM said he could still "turn it round" to win an historic fourth term in government.
The Sun cited Afghanistan as a key reason for abandoning Labour
He said voters would see that "the vision of the future that we have for Britain is about the hard working majority" - and that they would "see through" the Conservatives, who he claimed were like the Tory Party of the 1990s "that caused so much damage".
Mr Brown spoke as a YouGov poll for Sky News suggested Labour wer on 30%, compared with the Tories' 37% and Lib Dems' 21%, suggesting a boost from his speech on Tuesday.
The final day of conference will see speeches from Foreign Secretary David Miliband and Labour deputy leader Harriet Harman, who will attempt to lift the morale of activists with a closing address.
Meanwhile, General Stanley McChrystal, the US commander of NATO forces in Afghanistan, will give a speech in London on Thursday.
He has already told the US government that the situation is serious, but that success is achievable with the right strategy and additional resources.
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