President Barack Obama is to send 30,000 new troops to Afghanistan, pre-released excerpts from his new strategy speech say.
President Barack Obama will say they 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.
Mr Obama will deliver his speech in the next two hours.
The surge will bring US troop strength in Afghanistan to more than 100,000.
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.
A senior Pentagon official told the BBC ahead of the speech that the new troops, to be deployed by next summer, would be made up of 9,000 Marines and 21,000 regular soldiers, including trainers.
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.
'Quick punch'
Mr Obama left Andrews Air Force Base for West Point military academy in New York, where is due to make his announcement in a nationally televised speech to cadets.
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.
