David Miliband told delegates at the Nato Parliamentary Assembly that Britain would be "ready" to send more troops as part of an agreed strategy.
He said: "I, as much as anyone else, want to bring our troops back home to safety.
"But we cannot leave a vacuum which the Taliban will quickly fill, and under their umbrella, Al Qaida quickly follow.
"Counter-terrorism may deal with symptoms, it brings short-term success.
"But only a comprehensive strategy can deal with the causes and ensure that when we leave, we do so knowing that we will not have to return."
In his address to the assembly in Edinburgh, Mr Miliband concentrated on Nato's presence in Afghanistan.
He called for "political uplift" and backed calls for "burden-sharing".
Mr Miliband said: "We support the prosecution of a serious counter insurgency effort in Afghanistan.
"We do not see that as an alternative to counter-terrorism but as the best means to achieve it.
"And we are ready in the right conditions to raise our already high contribution on the basis of an agreed strategy."
The assembly of 28 member states - as well as delegates from Afghanistan and Pakistan - met for its 55th annual session at the Edinburgh International Conference Centre.
Mr Miliband began by highlighting the 472 casualties within the International Security Assistance Force (Isaf). Thousands more Afghan and Pakistani troops and civilians have been killed, he said.
He told delegates that the UK has suffered the "bloodiest war" since the Falklands conflict, with more than 100 deaths since Armistice Day last year.
Calling for allied support for Nato's operations, he said: "We will succeed in Afghanistan only if our military resources and development assistance are aligned behind a clear political strategy.
"Unless we get this right, our military will be able to suppress the cancers of insurgency and instability but not tackle their root causes."
Highlighting a recent BBC poll, he said: "Very few Afghans want the Taliban back, and that is our greatest strength. But they fear that the international community will tire of the war and the Taliban will return, inflicting brutal retribution on those who collaborated with the government."
Mr Miliband, who will visit Afghanistan later this week, said it was important to support President Hamid Karzai in efforts to bring opposing forces on side.
He added: "Some Afghan Taliban may be committed to global jihad but the vast majority are not. Their primary commitment is to tribe and locality.
"Our goal is not to fight to the death. It is to demonstrate clearly that they cannot win and to provide a way back into their communities for those who are willing to live peacefully."
Mr Miliband was later pressed on whether forces should remain in Afghanistan, or whether countries should withdraw their forces and concentrate on counter-terrorist work at home.
But the Foreign Secretary stressed the importance of keeping troops in the region.
He told the assembly: "If you're playing football and you want to make sure you don't concede any goals, the answer is not to put all of your players on the goal line - that's not a way to prevent the other side scoring goals.
"And I think in Afghanistan we have a similar challenge. If we believe simply by retreating within our own borders, by strengthening our own domestic measures, by improving our intelligence services that will be sufficient, I'm afraid I do not believe that would be sufficient as a strategy.
"What we need is an effective strategy with effective tactics in places like Afghanistan."
Nato secretary-general Anders Rasmussen spoke earlier to the assembly, urging countries to send more troops to bolster military efforts in Afghanistan.
He said: "To my mind it is obvious - that if we were to walk away and turn our backs on Afghanistan, al Qaida would be back in a flash.
"They would have a sanctuary from which to launch their strategy of global jihad, a strategy that is directed first and foremost against us.
"There's absolutely no reason to think otherwise and anyone who does so is not living in the real world."
He said Afghanistan was the "number one priority" and added: "I want to use this opportunity to strongly encourage you and your governments to make more military resources available."
He said nuclear-armed Pakistan would be under pressure if Nato troops "walk away".
Lord Robertson, a former secretary-general of Nato, backed Mr Rasmussen in his speech to the assembly.
The former defence secretary said: "We can, and I believe must, explain to the people in our countries what is going on in Afghanistan and why it matters to them and why we have to prevail."
He added: "It's in our hands, this mission to normalise Afghanistan and how we can make it safe for them, and indeed for our people at home. We can make it work or we can let it fail, that's what it comes down to."
He warned of the risk of abandoning Afghanistan, saying: "Let's be honest with ourselves - our people will never forgive this political generation if we were to walk away from the threat and horrors that we know would be the consequences of prematurely getting out of Afghanistan.
"So are we, the representatives of the people in every Nato country, going to lose our nerve when so much is at stake?
"Are we going to cut and run after all the lives that have been lost, all the blood that has been shed, all the life-changing injuries both physical and mental to those we sent out there to fight on our behalf? Are we going to cut and run away after all the money that has been spent?"
Lord Robertson added: "We know now and we knew ourselves in 2001 what dangers to the world came from the Taliban-controlled Afghanistan.
"And we know now, and it's staring us in the face, the dangers and the horrors that there would be in abandoning this sorely afflicted country to slide back to the Taliban."
About five protesters were stopped by police from trying to enter the conference centre before the session got under way this morning. They were led away in handcuffs.
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