Speaking to the young Marines preparing to head off to war, Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said the clock was ticking for the pivotal mission and that US-led forces had to break the momentum of the Islamist insurgents.
"We've got about 18 to 24 months," Mullen told a gathering of Marines at Camp Lejeune in North Carolina.
"The slope on this insurgency is going in the wrong direction and it has picked up three years -- each year -- to a significant degree," he said.
"We've got to turn that insurgency around."
Improving security in towns and villages would prepare the way for gradually handing over to Afghan forces, the admiral said.
The Marine unit from Camp Lejeune was among 16,000 forces who received deployment orders in the past few days as part of the surge.
Mullen and other top officials have acknowledged that US forces and their NATO-led allies will suffer more casualties as they take the fight to the Taliban, ousted in a 2001 US-led invasion.
After an exhaustive three-month strategy review, Obama unveiled his plan last week to send reinforcements while setting a July 2011 date for the start of a US drawdown.
During the White House deliberations, Obama pushed military advisers to come up with a faster plan to deploy forces in the first half of 2010, despite the daunting logistical challenge presented by Afghanistan's rugged terrain and lack of paved roads or runways.
"What he wanted was to pull this bell curve, which showed the deployment of forces, to the left," General David Petraeus said in an interview on Sunday.
"And we said that, yes, we thought we could indeed compress the time line for deployment," the general told "Fox News Sunday."
At Camp Lejeune, Mullen held up the surge two years ago in Iraq under former president George W. Bush as a model for success, saying the American military had learned how to fight a counter-insurgency campaign.
"We became from 2006 the best counter-insurgency force in the world," he said, adding that it happened "just in time."
Mullen said that ultimately the war against the Taliban would not be won in combat but by Afghans turning against the insurgents.
"In the long run, it's not going to be about killing Taliban because the Afghan people are going kick them out," he said.
A senior defense official meanwhile said Taliban insurgents have got to be "worried" over the surge in US and NATO troops as they had hoped to force allied forces to retreat.
The insurgents will be facing a larger NATO-led force just as they come under mounting pressure in neighboring Pakistan, where the government has launched offensives against the Taliban network, said Michele Flournoy, the undersecretary of defense for policy.
"In general they've got to be worried," she said.
The United States hopes the 7,000 additional troops promised by NATO allies will arrive in Afghanistan by the first half of 2010, Flournoy said.
Some of the allied contributions were troops that had deployed temporarily for the August presidential elections that now would be ordered to stay, but most of the additional forces were new, she said.
The new commitments are expected to swell the ranks of foreign troops in Afghanistan next year to 150,000, some two thirds of them US forces.
Apart from the 1,500 Marines heading to southern Helmand province this month, another 6,200 Marines from Camp Lejeune were due to deploy in the "early spring" next year and 800 Marines based in California would head to Afghanistan at the same time, the Pentagon said.
The US Army also was mobilizing troops, with 3,400 from the 10th Mountain Division in Fort Drum in New York state ordered to head out in the spring to focus on training Afghan security forces.
About 4,100 support troops, which include medics and bomb disposal teams, also were ordered to deploy between now and the spring of 2010.
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