WASHINGTON—Defense Secretary Robert Gates has decided to send an additional 1,400 Marine combat forces to Afghanistan, officials said, in a surprise move ahead of the spring fighting season to try to cement tentative security gains before White House-mandated troop reductions begin in July.

U.S. commanders in Afghanistan face intense pressure to show sustainable security gains in the first half of 2011. Military officials fear an upswing in attacks by the Taliban in the spring could convince the White House that the Pentagon's war strategy is flawed and that the troop pullout—the details of which have yet to be ironed out—should be accelerated.