Washington  - Withdrawing US forces from Afghanistan is not among the options under consideration by US President Barack Obama as he outlines a new strategy for the conflict.

"I don't think we have the option to leave," White House spokesman Robert Gibbs said Monday. "I think that's quite clear."

Obama is in an ongoing strategic review, assessing various possibilities for the war in Afghanistan, which is entering its ninth year. The top US commander there, General Stanley McChrystal, is reportedly seeking an additional 40,000 troops.

There are already more than 60,000 US soldiers on the ground, and Obama is facing resistance from his own centre-left Democrats in Congress over deploying more forces.

McChrystal warned in a speech last week in London of failure in Afghanistan unless more troops are sent, prompting national security adviser James Jones to say on television over the weekend that it would be better if military leaders kept their conclusions within the chain of command.

US Defence Secretary Robert Gates, speaking late Monday to students at George Washington University with Secretary of State Hillary Rodham Clinton, acknowledged that if Taliban insurgents are allowed to recapture large areas of Afghanistan, al-Qaeda could use the territory to "empower itself."

Gates said it could improve the Taliban and al-Qaeda's ability to grow in stature, recruit new jihadists and raise money.

Clinton and Gates refused to discuss the options under consideration for moving forward in Afghanistan, citing the need to keep the advice confidential until Obama reaches a decision.

Obama will meet Tuesday with congressional leaders to discuss the process and will hold another meeting later this week with his top advisors. The White House has said it will be weeks before any final decisions are made. (dpa)