Foreign Secretary David Miliband has said the international presence in Afghanistan is essential to prevent the country from becoming a failed state.
Mr Miliband said Nato troops have made "an enormous difference"
He told the BBC the risk of it becoming a haven for terrorists again meant there was a security, as well as a moral, basis for UK troops' presence.
Mr Miliband also rejected calls for President Hamid Karzai to be removed.
He was responding to comments by Lord Ashdown that time was running out for the country.
Lord Ashdown asked if the international community was "losing the battle".
But Mr Miliband said "we are not creating a colony" and had backed the democratic elections which saw Afghans electing Mr Karzai.
Mr Miliband, who visited Afghanistan last week with US Secretary of State Condoleezza Rice rejected suggestions Afghanistan was a failed state.
A pull-out by UK and international community troops would "precipitate even more dangerous insecurity", he added.
Lord Ashdown told the BBC: "Afghanistan is a failed state, I don't think it is on the edge of it.
"The question is: Are we on the edge of losing this battle? Events are now moving against us and the tide has to be turned and you have not got very long to do it."
US Defense Secretary Robert Gates warned the mission could eventually "destroy" Nato if it led to a two-tiered alliance of those "who are willing to fight and those who are not".
Mr Miliband said: "There are big challenges, a big test for the international community, a big test also for the Afghan government, but I think we are both sober about the challenges there but also determined in the belief that it is important that we are there to make a difference.
"Without the international presence then Afghanistan certainly would be a failed state.
"I don't believe it is a failed state - there are five million kids in school, five million refugees have come back to the country, two-thirds of the country now gets healthcare. Those are all important aspects of progress."
Mr Miliband, who said he was meeting Defence Secretary Des Browne on Monday, added: "Troops alone are not going to be the answer. It has to be the building-up of a decent society in Afghanistan that is able to cater for its own affairs."
He said people were "right to ask hard questions" but said: "The difference is being made every day - by our troops and by our diplomats and by our aid workers in Afghanistan."
On the question of wider international involvement in Afghanistan, Mr Miliband said: "We do need the whole of the international community, including European countries, to step up not just in terms of troops.
"We have also a massive issue to do with the Afghan police force which is critical to security in that country.
"So there are a range of ways in which the international community needs to make its presence felt, including European countries."
ISAF REGIONAL COMMANDS AND RECONSTRUCTION TEAMS
Countries contributing more than 1,000 troops (6 February 2008):
Australia - 1,070
Germany - 3,210
Italy - 2,880
Netherlands - 1,650
Poland - 1,100
UK - 7,800
US - 15,000