The additional troops would help combat an expected offensive from the Taleban.

If the marines are sent, they would reinforce Nato troops in southern Afghanistan, the Pentagon said.

The US already has about 26,000 troops in Afghanistan, half of them under the Nato-led International Security Assistance Force.

Growing insurgency

Pentagon spokesman Geoff Morrell said the proposal would go before Mr Gates on Friday but a decision is not expected immediately.

Commanders in Afghanistan have been saying for months that they need an additional 7,500 troops to fight the Taleban insurgency that has grown over the last two years.

Mr Gates has been pressing Nato allies for months to provide the troops but they have not been forthcoming.

"The commanders need more forces there. Our allies are not in the position to provide them. So we are now looking at perhaps carrying a bit of that additional load," Mr Morrell said.

He said the idea was to get the troops in place to prevent a Taleban offensive after the winter snows melt.

"The timing is that they would be in place by April, for a one-time seven month deployment".

The force under consideration for the deployment would include a marine air-ground task force of 2,200 combat troops and a battalion of marines to help train Afghan soldiers.