The proposal has not yet been presented to Defense Secretary Robert M. Gates, whose very public efforts in recent weeks to get NATO nations to increase troop commitments in Afghanistan have yet to succeed.

If approved, most of the 3,200 troops, all marines, would be sent to southern Afghanistan, where NATO forces have faced a growing Taliban resurgence.

Gen. Dan K. McNeill, commander of NATO forces in Afghanistan, requested the additional forces, and the idea has been endorsed by Adm. William J. Fallon, commander of American forces in the Middle East, and by Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, Pentagon officials said.

The proposal for more troops was first reported Wednesday by ABC News on its Web site.

The security assistance mission has about 40,000 troops, of which 14,000 are American. An additional 7,500 combat soldiers and security trainers, as well as some helicopters, must be added to meet force levels that NATO nations have agreed are needed. Separately, the United States has 12,000 other troops in Afghanistan conducting counterterrorism missions. “This proposal is pre-emptive,” said Geoff Morrell, the Pentagon press secretary. “It does not reflect a deterioration in the security situation.”

Commanders in Iraq, he said, requested the additional marines to ensure that NATO and Afghan troops would be ready for any attacks from the Taliban — anticipated after the spring thaw.

The new Marine Corps mission in Afghanistan would be in addition to the service’s duties in Iraq, and not a replacement of that combat role, he said.

The Bush administration is deeply concerned about Afghanistan, and last month began top-to-bottom reviews of the mission, from security and counterterrorism to political consolidation and economic development.

Commanders in Afghanistan would like even more troops, but that is unlikely because the American military is so strained by current deployments there and in Iraq, officials said. So Mr. Gates will continue pressing NATO nations.