WASHINGTON — President Barack Obama plans to send 30,000 more troops to Afghanistan over six months, a senior administration official said Tuesday, on an accelerated timetable that would dispatch several hundred Marines by Christmas.

With the full complement of troops expected by next summer, the heightened pace of Obama’s military deployment in the 8-year-old war would appear to match the 2007 troop surge in Iraq, which rushed 20,000 combat forces to quell violence there. The Afghan surge would similarly aim to reverse Taliban gains and secure population centers in the volatile south and east parts of the country.

In his prime-time speech to the nation Tuesday night at West Point, Obama will tie the escalation to an exit strategy, laying out a rough timeframe and some dates for when the main U.S. military mission would end. Public opinion in this country has become increasingly divided over American participation in the stalemated war.

Obama will try to sell a skeptical public on his bigger, costlier war plan by coupling the large new troop infusion with an emphasis on stepped-up training for Afghan forces that he says will allow the U.S. to leave.

The new infusion of troops had been envisioned to take place over a year, or even more, because force deployments in Iraq and elsewhere make it logistically difficult, if not impossible, to go faster.

Instead, Obama directed his military planners to make the changes necessary to speed up the Afghanistan additions, said the official, who spoke on condition of anonymity because the details had not yet been announced.

Military officials said at least one group of Marines is expected to deploy within two or three weeks of Obama’s announcement, and would be in Afghanistan by Christmas. Larger deployments wouldn’t be able to follow until early in 2010.

The initial infusion is a recognition by the administration that something tangible needs to happen quickly, officials said. The quick addition of Marines would provide badly needed reinforcements to those fighting against Taliban gains in the southern Helmand province, and could lend reassurance to both Afghans and the U.S. public.

Obama also will insist that a specific withdrawal scenario be built into the process of adding new forces.

White House press secretary Robert Gibbs said in a TV interview Tuesday that Obama would lay out an endgame for U.S. involvement.

“We want to — as quickly as possible — transition the security of the Afghan people over to those national security forces in Afghanistan,” he told ABC’s “Good Morning America.” ”This can’t be nation-building. It can’t be an open-ended, forever commitment.”