About 3,200 extra United States marines are to be sent to Afghanistan this spring, the Pentagon has confirmed.
The US already has about 26,000 troops in Afghanistan
Of the marines, 2,200 will help Nato fight the Taleban in the south of the country, a statement said. They will be deployed for about seven months.
The other 1,000 troops will help train Afghan security forces.
The US already has about 26,000 troops in Afghanistan, half of them under the Nato-led International Security Assistance Force (Isaf).
Commanders in Afghanistan have been saying for months that they need extra troops to fight the Taleban insurgency that has grown over the past two years.
Last week, Pentagon spokesman Geoff Morrell said: "Our allies are not in the position to provide them. So we are now looking at perhaps carrying a bit of that additional load."
He said the idea was to get the troops in place to prevent a Taleban offensive after the winter snows melt.
Tuesday's Pentagon statement said the "extraordinary, one-time deployment" of marines would temporarily meet an Isaf request for more troops in southern Afghanistan.
"As a member of the Nato alliance, the United States is doing its part to ensure Isaf has the forces necessary to ensure the hard-fought gains accumulated during the past six years by the Afghans and Nato are made irreversible," the Pentagon statement said.
It said it would work to ensure "the need for future extraordinary deployments by the US or any ally and partner is minimised".
US-led forces toppled the Taleban government in late 2001.