"There's no way to defeat al-Qaeda, which is the mission, with just that approach - you can't do it remotely, you can't do it offshore."
US 'tired of war'
Gates said: "The nation has been at war for eight years; the fact that Americans would be tired of their sons and their daughters at risk and in battle is not surprising.
But, he added: "I don't believe that the war is slipping through the administration’s fingers."
Marwan Bishara, Al Jazeera's senior political analyst, said that US officials are facing a challenge in convincing the domestic public that the war is worth fighting, given that 2009 is heading towards being the deadliest year so far for foreign troops serving in Afghanistan.
"What has been problematic over the last two or three months - despite the radical augmentation of tens of thousands of American soldiers - is that the situation in Afghanistan continued to escalate and the allied forces, especially the American forces, have been hit worse," he said.
"Contrary to what they say, the US military will have to get more violent in Afghanistan in order to reach some of their objectives.
"Obama called Iraq the 'stupid war' and Afghanistan the 'right' war. Well, guess what? Fifty per cent of Americans think the 'right' war is going the wrong way.
"Mullen and Gates realise that, unless they rush to to change the tide in Afghanistan, the support for the war in the US will diminish."
The pair's comments came amid expectations that general Stanley McChrystal, the commander of US and Nato forces in Afghanistan, will ask for more troops and money for the conflict in the coming weeks.
McChrystal already has 103,000 troops under his command, including 63,000 from the US.
He will have 68,000 by the end of the year, in line with an order by Barack Obama, the US president, to send more troops to the country.
Gates said he would consider any request for more troops and resources made by McChrystal.
"I'm very open to the recommendations and certainly the perspective of General McChrystal," Gates said, referring to McChrystal's recent report on the progress of the war.
Gates has in the past said that sending too many US and Nato soldiers to Afghanistan could cause Afghans to see it as an occupying force.
But he said that those concerns could be "mitigated" if any extra US troops "interact with the Afghans in a way that give confidence to the Afghans that we're partners and their allies".
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