President Barack Obama is expected to decide within days how many extra U.S. troops to send to Afghanistan, the U.S. defense chief said on Tuesday.

Defense Secretary Robert Gates also suggested the initial deployment could be as small as one combat brigade, as the Obama administration moves to complete its review of Afghan strategy before NATO holds its next summit in April.

"The president will have several options in front of him and I think he will make those decisions probably in the course of the next few days," Gates told reporters at a Pentagon briefing.

Up to now, Obama had been widely expected to announce the deployment of three combat units encompassing up to 17,000 U.S. forces as part of a plan to nearly double the U.S. military presence to about 60,000 troops over the next 12 to 18 months.

U.S. military planners have considered sending two Army combat brigades, each with about 3,500 soldiers, and a larger Marine task force that could number close to 10,000 troops.

But Pentagon officials have sounded a more cautious note in recent days, amid signs that the Obama White House would devote more time to deliberation over the new president''s first major deployment of U.S. forces overseas.

"It seems to me a thoughtful and deliberative approach to that decision is entirely appropriate," Gates said.