Robert Gates and Mike Mullen both warned of limited time in Afghanistan
US Defence Secretary Robert Gates has defended American military plans in Afghanistan, but warned that there is now "limited time" to show success.
Only now does the US have sufficient resources on the ground to start implementing President Barack Obama's new Afghan strategy, Mr Gates said.
His comments come days after the top US general in Afghanistan called for a fresh approach to the conflict.
Gen Stanley McChrystal said success was possible, but strategy must change.
Days after receiving Gen McChrystal's detailed report into the Afghan conflict, Mr Gates spoke in defence of Mr Obama's approach to Afghanistan at a Pentagon news conference with Admiral Mike Mullen, chairman of the Joint Chiefs of Staff.
Later on Friday British Prime Minister Gordon Brown is due to restate his commitment to Afghanistan in a major speech.
Speaking at the Pentagon, Mr Gates said firmly: "I don't believe that the war is slipping through the administration's fingers."
"I absolutely do not think it is time to get out of Afghanistan," he said, but cautioned that there remains "limited time for us to show that... this approach is working".
UK AND US OPINION ON WAR
In UK, 58% believe the war is not winnable, 31% disagree. ComRes/Independent. July 09
In US, 42% think America is winning the war, 36% think it is losing. ABC News/Washington Post. Aug 09
54 British troops killed in 2009 so far, compared with 47 in 2008
172 US troops killed in 2009 so far, compared with 155 in 2008
It was another sober assessment of America's mission in Afghanistan, says the BBC's Washington correspondent, Paul Adams.
Alongside his civilian boss, Adm Mullen repeated the charge that for years the mission in Afghanistan was poorly funded and staffed.
"Part of why it has gotten more serious and has deteriorated has been directly tied to that," he said.
But the situation could be turned around in the next 12 to 18 months, he said.
Our correspondent says Mr Obama's administration knows that it faces an uphill struggle on the battlefields of Afghanistan and Washington.
Congressmen are growing restive about the duration and cost of this campaign, and polls show that support for the war among Americans is on the wane.
Speaking of the possibility of increasing the number of US troops even further, Mr Gates said he would only consider that option if Afghans felt reassured by a larger military presence.
"If they interact with the Afghans in a way that gives confidence to the Afghans that we're their partners and their allies, then the risks that I have been concerned about the footprint becoming too big and the Afghans seeing us in some role other than partners I think is mitigated."
He reiterated that the danger posed by al-Qaeda and the Taliban was the main reason for the continuing US presence in Afghanistan.
That message chimes with that due to be delivered in the UK on Friday.
In a speech on the war, which has claimed 212 British military lives since 2001, Prime Minister Mr Brown will say: "When the security of our country is at stake we cannot walk away."
He will also say that financial and logistical support is being increased for the British effort.
In a strategic assessment sent to US leaders in August but yet to be published officially, Gen Stanley McChrystal, who was appointed as US commander in Afghanistan earlier this year, called for a new approach.
"The situation in Afghanistan is serious, but success is achievable and demands a revised implementation strategy, commitment and resolve, and increased unity of effort," Gen McChrystal wrote.