by Helene Cooper

WASHINGTON - President Obama will send an additional 17,000 American troops to Afghanistan this spring and summer in the first major military move of his presidency, White House officials said on Tuesday.

The increase would come on top of 36,000 American troops already there, making for an increase of nearly 50 percent. In issuing the order, Mr. Obama is choosing a middle ground, addressing urgent requests from commanders who have been pressing for reinforcements while postponing a more difficult judgment on a much larger increase in personnel that the commanders have been seeking.

In a written statement issued by the White House on Tuesday evening, Mr. Obama said that deteriorating security in Afghanistan demands "urgent attention and swift action" to address a problem that has not received the strategic attention, direction and resources it urgently requires."

White House officials said that 8,000 Marines from Camp Lejeune, N.C., will deploy in the next few weeks, aiming to be on the ground in Afghanistan by late spring, while an Army brigade from Fort Lewis, Wash., composed of 4,000 soldiers, will deploy in the summer.

An additional 5,000 Army support troops and so-called "enablers" will also be deploying in the summer, administration officials said, which will bring the number of troops deployed as part of this presidential order to 17,000. The decision does carries some political risks for Mr. Obama, whose election was interpreted by many Americans as a mandate to bring troops home from Iraq. But Mr. Obama has now announced additional American troops are headed to Afghanistan before he has withdrawn any troops from Iraq.

But White House officials said both of the units being sent to Afghanistan were originally supposed to be going to Iraq.

"We have the ability to do this because we will be drawing down in Iraq," a senior White House official said.

Mr. Obama is under pressure from his military commanders in Afghanistan, who have been pressing for reinforcements of about 30,000 soldiers, almost twice as many as the president has so far decided to send. The commanders hope to have additional forces in place by late spring or early summer as part to help counter growing violence and chaos in the country, particularly in advance of the upcoming presidential elections, which are expected to take place in August.

Mr. Obama will still have to make a decision on the additional troops that are part of Gen. David D. McKiernan's standing request. Defense officials say that Mr. Obama cannot satisfy the full request from Gen. McKiernan, the top American commander in Afghanistan, without withdrawing a substantial number of forces from Iraq.

Richard Holbrooke, Mr. Obama's special representative to Afghanistan and Pakistan, who is on his way home from his first trip to the region, is helping to conduct the administration's review of policy in Afghanistan. Administration officials say the review needs to be completed before Mr. Obama makes his first overseas trip as president, when he attends the NATO summit in France and Germany in April.

Mr. Obama is expected to press America's European allies at the summit for additional troops for Afghanistan, along with more development help.