Some of the Marines would be sent directly into combat in Taliban strongholds in southern Afghanistan; the rest would help train the Afghan army and police, reports CBS News national security correspondent David Martin.
The move represents a shift in Pentagon thinking that has been slowly developing after months of repeated insistence that the U.S. was not inclined to fill the need for as many as 7,500 more troops that commanders have asked for there. Instead, Defense Secretary Robert Gates pressed NATO allies to contribute the extra forces.
The new plan was drawn up after NATO countries failed to respond to the request for more troops to contain Taliban forces operating out of sanctuaries in Pakistan, adds Martin.
Pentagon Press Secretary Geoff Morrell said Wednesday that a proposal will go before Gates on Friday that would send a ground and air Marine contingent as well as a Marine battalion - together totaling more than 3,000 forces - to southern Afghanistan for a "one-time, seven-month deployment."
That would bring the overall number of U.S. troops in Afghanistan to 30,000, reports Martin.
Gates, he said, will want to review the request, and is not likely to make a final decision on Friday.
"He will take it and consider it thoroughly before approving it," said Morrell. "I just want to get people away from the idea that this is going to be imminently approved by the secretary."
He said Gates "has some more thinking to do on this matter because it's a serious allocation of forces."
Morrell added that Gates' thinking on the issue has "progressed a bit" over time as it became clear that it was politically untenable for many of the NATO nations to contribute more combat troops to the fight.
"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," the spokesman said.
Morrell said the move was aimed at beating back "another Taliban offensive," that is expected this spring - as has occurred in previous years.
When Gates was in Afghanistan last month, commanders made it clear they needed the additional forces.
Last year was the most violent since the U.S.-led invasion of Afghanistan in 2001. The number of attacks has surged, including roadside bombings and suicide assaults.
Currently there are about 27,000 U.S. troops in Afghanistan, including 14,000 with the NATO-led coalition. The other 13,000 U.S. troops are training the Afghan forces and hunting al Qaeda terrorists.