Support for the Afghan mission among the US public has been falling
President Barack Obama is to send 30,000 new US troops to Afghanistan, pre-released excerpts from his new strategy speech say.
President Obama will say the troops will be deployed "in the first part of 2010 - the fastest pace possible".
Mr Obama is also spelling out key elements of an exit plan, including beginning to hand over to Afghan security forces by mid-2011.
The president will deliver his speech in the next two hours.
The surge will bring US troop strength in Afghanistan to more than 100,000.
Appeal to allies
Mr Obama will deliver his televised speech shortly to cadets at the West Point military academy in New York.
According to the excerpts from the speech released in advance by the White House, he will argue that the surge will help "create the conditions for the United States to transfer responsibility to the Afghans".
He will also call on America's allies to boost their troop commitment.
"Some have already provided additional troops, and we are confident that there will be further contributions in the days and weeks ahead," he is due to say.
"Our friends have fought and bled and died alongside us in Afghanistan. Now, we must come together to end this war successfully. For what's at stake is not simply a test of Nato's credibility - what's at stake is the security of our Allies, and the common security of the world."
Mr Obama will say that the US will take the Iraq experience as its model for withdrawal.
"Just as we have done in Iraq, we will execute this transition responsibly, taking into account conditions on the ground," he will say.
America, he will add, "will continue to advise and assist Afghanistan's security forces to ensure that they can succeed over the long haul".
"But it will be clear to the Afghan government - and, more importantly, to the Afghan people - that they will ultimately be responsible for their own country," the US president will say.
Mr Obama's speech is likely to give a sense that the troops are going in hard to deal with the Taliban, then looking to get out again as soon as possible, says the BBC's North America editor, Mark Mardell.
In a country suspicious of sending more troops, where the word Vietnam still resonates, it is important that there is no lingering sense of a conflict that will drag on, he adds.
Mr Obama has reached his deployment decision after more than three months of deliberations and 10 top-level meetings with advisers.
It is the second time he is increasing the US contingent in Afghanistan after a reinforcement of 17,000 troops in February.
Some 32,000 other foreign troops are serving in Afghanistan but Nato allies have been cautious about contributing further forces.
The US president outlined his new strategy to Afghan counterpart Hamid Karzai in an hour-long video conference on Tuesday morning.
He told Mr Karzai that US efforts in Afghanistan were not "open-ended" and would be measured against goals over a two-year period, the White House said.
Rising violence - more than 900 US soldiers have died in Afghanistan - and August's discredited elections have fanned mounting domestic opposition to the eight-year-old war.
Earlier this year, the US military commander in Afghanistan, Gen Stanley McChrystal, warned America risked failure unless troop numbers were increased. He requested 40,000 more soldiers.
The BBC's Martin Patience in Kabul says that while the speech will probably receive a cautious welcome from the Afghan government, many people in the country do not want any more foreign forces.
They say every time America sends more troops the security situation gets worse, and some question why the US is spending billions of dollars on the military - and not on aid and reconstruction, our correspondent says.
France said it might be able to spare some personnel to train Afghan forces
Nato officials said on Tuesday that President Obama had asked European allies to contribute between 5,000 and 10,000 new troops to Afghanistan.
But President Nicolas Sarkozy has ruled out deploying more soldiers, although he might send military trainers, his special envoy to Afghanistan told AFP news agency.
French Defence Minister Herve Morin said earlier: "If there were to be an additional effort, the only effort that would make sense would be in terms of Afghan army and police training."
In Berlin, Chancellor Angela Merkel told a news conference Germany would wait until after a 28 January conference in London on Afghanistan before deciding on any troop increases.
On Monday, Britain confirmed it was sending 500 more troops, taking the UK's total deployment to 10,000.
Italy has also said it will increase its force, although without saying by how much.