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.