WASHINGTON, D.C. — President Barack Obama plans to send 30,000 more troops to Afghanistan over six months, an accelerated timetable — with an endgame built in — that would have the first Marines there as early as Christmas, a senior administration official told The Associated Press.

With the full complement of new troops expected to be in Afghanistan by next summer, the heightened pace of Obama’s military deployment in the 8-year-old war appears to mimic the 2007 troop surge in Iraq, a 20,000-strong force addition under former President George W. Bush.

Similar in strategy to that mission, Obama’s Afghan surge aims to reverse gains by Taliban insurgents and to secure population centers in the volatile south and east parts of the country.

In a prime-time speech to the nation Tuesday night from West Point that ends a 92-day review, Obama will seek to help sell his much bigger, costlier war plan by tying the escalation to an exit strategy, said the official who spoke on condition of anonymity.

By laying out a rough timeframe and some dates for when the main U.S. military mission would end, as well as emphasizing stepped-up training for Afghan forces, the president was acknowledging the increasingly divided public opinion over continued American participation in the stalemated war.

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

With U.S. casualties in Afghanistan sharply increasing recently and little sign of progress, the war Obama once liked to call one “of necessity,” not choice, has grown less popular with the public and within his own Democratic party.

In recent days, leading Democrats have talked of setting tough conditions on deeper U.S. military involvement, or even staging outright political opposition.

The displeasure on both sides of the aisle was likely to be on display when congressional hearings on Obama’s strategy get under way later in the week on Capitol Hill.

In his speech and in meetings overseas in the coming days, Obama also will ask NATO allies to contribute more — between 5,000 and 10,000 new troops — to the separate international force in Afghanistan, diplomats said.

One official from a European nation said the troop figure was included in an official NATO document compiled on the basis of information received from Washington ahead of Obama’s announcement.

The NATO force in Afghanistan now stands at around 40,000 troops.

The 30,000 new U.S. troops will bring the total in Afghanistan to more than 100,000 U.S. forces by next summer. New infusions of U.S. Marines will begin moving into Afghanistan almost as soon as Obama announces a redrawn battle strategy.

The president’s long-awaited troop increase 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.

But Obama directed his military planners to make the changes necessary to hasten the Afghanistan additions, said the official, who declined to be publicly identified because the formal announcement of details was still pending.

Officials were not specific on the withdrawal date that Obama has in mind nor the changes the military will be required to make to get the troop deployments into Afghanistan on the president’s new, speedier timeline.

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.

This initial infusion is a recognition by the administration that something tangible needs to happen quickly, military officials said.

The new Marines would provide badly needed reinforcements to those fighting against Taliban gains in the southern Helmand province. They also could lend reassurance to both Afghans and a war-weary U.S. public.

Obama’s announcement comes near the end of a year in which the war has worsened despite the president’s infusion of 21,000 forces earlier this year.

He began rolling out his decision Sunday night, informing key administration officials, military advisers and foreign allies in a series of private meetings and phone calls that stretched into Monday.

Previewing a narrative the president is likely to stress, Gibbs told ABC that the number of fresh troops don’t tell the whole story. Obama will emphasize that Afghan security forces need more time, more schooling and more U.S. combat backup to be up to the job on their own.

“We’re going to accelerate going after al-Qaida and its extremist allies,” Gibbs said. “We’ll accelerate the training of an Afghan national security force, a police and an army.”

In Kabul, Lt. Gen. William B. Caldwell, the new head of a U.S.-NATO commandresponsible for training and developing Afghan soldiers and police, said Tuesday that although the groundwork is being laid to expand the Afghan National Army beyond the current target of 134,000 troops, to be reached by Oct. 31, 2010, no fixed higher target is set.

There is a notional goal of eventually fielding 240,000 soldiers and 160,000 police, but Caldwell said that could change.

“Although that is a goal and where we think it could eventually go to, it’s not a hard, firm, fixed number,” he said in a telephone interview with The Associated Press.

He indicated that one reason for avoiding a hard-and-fast commitment to those higher numbers is the expected cost.

So his orders are to reach the targets of 134,000 soldiers and 96,800 police by next October. He intends to hold annual reviews, beginning next spring or early summer, to determine whether the notional higher targets of 240,000 soldiers and 160,000 police — for a combined total of 400,000 by 2013 — are still the right goals for Afghanistan.

“If you grow it up to 400,000 — if you did grow all the way to that number, and if it was required to help bring greater security to this country — then of course you have to sustain it at that level, too, in terms of the cost of maintaining a force that size,” he said.

Nearly all the cost of building Afghan forces has been borne by the U.S. and other countries thus far.

Obama also will make tougher demands on the governments of Pakistan and, especially, Afghanistan.

The Afghan government said Tuesday that President Hamid Karzai and Obama had an hourlong video conference. Obama was also going to speak with Pakistani President Asif Ali Zardari.

In Afghanistan, rampant government corruption and inefficiency have made U.S. success much harder. Obama was expected to place tough conditions on Karzai’s government.

Obama was spending much of Monday and Tuesday on the phone, outlining his plan — minus many specifics — for the leaders of France, Britain, Germany, Russia, China, India, Denmark, Poland and others. He also met in person at the White House with Australian Prime Minister Kevin Rudd.

A briefing for dozens of key lawmakers was planned for Tuesday afternoon, just before Obama was set to leave the White House for the speech against a military backdrop.

• Anne Gearen and Jennifer Loven