President Obama has ordered a significant troop increase in Afghanistan authorizing the deployment of an additional 17,000 troops to take on the resurgent Taliban in southern Afghanistan. The first units will begin deploying in the spring.
In a statement released by the White House, Obama announced his approval of a request from the top U.S. military commanders in Afghanistan for additional troops "to meet urgent security needs."
The president has approved sending a large Marine unit known as an expeditionary brigade to Afghanistan consisting of 8,000 Marines and an Army Stryker combat brigade of 4,000 soldiers. The additional troops will consist of combat support units known as enablers and will receive their orders at a later date.
Gen. David McKiernan, the top U.S. commander in Afghanistan, had requested as many as four additional combat brigades, a request that equaled 30,000 troops when the enabler troops were added to the mix.
Meeting that request completely could increase the size of the overall U.S. force in Afghanistan to 60,000 troops from the current number of 38,000.
Prior to today's announcement, the Pentagon had already sent one of those brigades to Afghanistan along with 2,000 enabler troops. An additional aviation brigade is slated to arrive in March. Combined with today's announcement, that would mean the bulk of the troop request for Afghanistan has been met.
A Defense Department official said the administration may consider sending the balance of the additional troop request after the conclusion of the administration's various strategy reviews under way for Afghanistan and Pakistan.
Obama said his decision to send more troops to Afghanistan does not "pre-determine the outcome" of the strategy review for that country.
"Instead, it will further enable our team to put together a comprehensive strategy that will employ all elements of our national power to fulfill achievable goals in Afghanistan," Obama said. "As we develop our new strategic goals, we will do so in concert with our friends and allies as together we seek the resources necessary to succeed."
A Marine official said the 2nd Marine Expeditionary Brigade based in Camp Lejeune will begin deploying in April and May of this year. The majority of the Marine combat units in the brigade were originally slated to serve in Iraq as replacement troops. The Marine brigade will bring its own aviation and support units, which accounts for its large size.
The Army Stryker brigade that will deploy over the summer was also originally intended to head to Iraq.
McKiernan said he requested a Stryker brigade because he feels the light armored vehicles will be able to provide greater access to hard-to-reach places while also providing greater protection from the growing threat of roadside bombs in Afghanistan.
The flow of additional forces actually began in January when the Third Brigade, 10th Mountain Division began to deploy into two provinces south of Kabul, the capital of Afghanistan.
However, meeting the request for the additional three brigades into Afghanistan proved difficult for Pentagon planners, as it seemed contingent on a corresponding U.S. troop drawdown in Iraq that might develop over the next year and a half.
But last month, Defense Secretary Robert Gates told Congress that a faster timeline might be in the works.
"Should the president make the decision to -- the final decision to deploy additional brigades to Afghanistan, we could have two of those brigades there probably by late spring, and potentially a third by mid-summer," Gates said.
ABC News' Jake Tapper and Sunlen Miller contributed to this report.