President Barack Obama has authorised the deployment of up to 17,000 extra US troops to Afghanistan, saying they are to "meet urgent security needs".
Two brigades - one army and one marine - are to be sent, Mr Obama said in a statement released by the White House.
This first major troop deployment under Mr Obama comes as his administration reviews US policy in Afghanistan.
He has vowed to focus US military efforts on fighting the Taleban in Afghanistan, rather than on Iraq.
The additional troops to be sent to Afghanistan had originally been scheduled to go to Iraq.
"The fact that we are going to responsibly drawdown our forces in Iraq allows us the flexibility to increase our presence in Afghanistan," Mr Obama said.
A senior White House official was quoted by Reuters as saying that a decision on cutting US troop levels in Iraq would be taken within "weeks".
During his election campaign Mr Obama promised to withdraw US combat troops from Iraq within 16 months after becoming president.
The additional troops are to be sent before warmer weather brings an expected increase in fighting in Afghanistan, US defence officials said.
The deployment will be made up of 8,000 marines, and 4,000 army soldiers, plus another 5,000 support staff. They will serve in the south of Afghanistan, where the violence has been worst.
"The 2nd Marine Expeditionary Brigade from Camp Lejeune, North Carolina, with approximately 8,000 marines will deploy to Afghanistan in late spring 2009," said a statement from the Pentagon.
"The 5th Stryker Brigade, 2nd Infantry Division from Ft Lewis, Washington, will deploy approximately 4,000 soldiers to Afghanistan in mid-summer 2009.
"Approximately 5,000 additional troops to support these combat forces will receive deployment orders at a later date."
The US already has about 14,000 troops serving with a Nato-led mission. There are also 19,000 US troops under sole US command charged with fighting Taleban and al-Qaeda insurgents.
US commanders in Afghanistan have requested 30,000 additional troops.
The increase "is necessary to stabilise a deteriorating situation in Afghanistan", Mr Obama said.
"The Taleban is resurgent in Afghanistan, and al-Qaeda supports the insurgency and threatens America from its safe-haven along the Pakistani border," he said.
Afghanistan, he said, "has not received the strategic attention, direction and resources it urgently requires".
"I recognise the extraordinary strain that this deployment places on our troops and military families," the president said.
The announcement came after the UN said that the number of civilians killed in the conflict in Afghanistan rose by 39% last year.
Militants were to blame for 55% of the 2,118 civilian deaths, while US, Nato and Afghan forces were responsible for 39%, according to the UN report - the highest number since the Taleban were ousted in 2001.