"This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires," Obama said in a statement.
That was a slap at his predecessor, former President George W. Bush, whom Obama has accused of slighting urgent national security needs in Afghanistan in favor of war in Iraq.
The White House said the new commander in chief would send a Marine unit and one additional Army brigade to Afghanistan this spring and summer. About 8,000 Marines are expected to go first, followed by an Army brigade, totaling about 4,000 troops, and 5,000 support forces. The United States has slightly more than 30,000 troops in the country now.
Obama's decision shifts the Army brigade and several thousand Marines from already approved deployments to Iraq later this year to new destinations in southern Afghanistan. As a result, the number of combat brigades in Iraq will likely drop from 14 to 12 by the summer, unless other units are identified to fill those slots.
A decision on that troop cut in Iraq has not yet been made, but Obama campaigned on a pledge to bring combat troops out of Iraq.
The new troops represent the first installment on a larger influx of U.S. forces widely expected this year. Obama's move would put several thousand troops in place in time for the increase in fighting that usually occurs with warmer weather and ahead of national elections in August.
The additional forces partly answer a standing request from the U.S. commander in Afghanistan, Gen. David McKiernan, who has sought as many as 30,000 additional U.S. troops to counter the resurgence of the Taliban militants and protect Afghan civilians.
"There is no more solemn duty as president than the decision to deploy our armed forces into harm's way," Obama said. "I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action."
The new units are the 2nd Marine Expeditionary Brigade from Camp Lejeune, N.C., and the 5th Brigade, 2nd Infantry Division, an Army Stryker brigade from Fort Lewis in Washington state.
The 3,900 soldiers from Fort Lewis had been scheduled to go to Iraq this summer. Fort Lewis spokesman Joe Piek said the 5th Stryker Brigade is the Army's newest Stryker Brigade - activated in May 2007 - and the fourth to be formed, equipped and trained at Fort Lewis. The brigade's soldiers are currently at the Army National Training Center in Fort Irwin, Calif., for exercises through the end of February.
The Pentagon outlined an addition of 12,000 combat forces, and said 5,000 support troops would be identified later. A Marine Expeditionary Brigade can vary in size and makeup.
Among the forces recently notified of deployment is a Marine unit of infantry and ground troops from Camp Pendleton in southern California, said Kurt Bardella, a spokesman for Rep. Darrell Issa, a Republican who represents the congressional district where the base is located.
Obama's plan to withdraw troops from Iraq allows him to increase the numbers in Afghanistan. Last fall, the Pentagon announced that the Fort Lewis brigade was being ordered to go to Iraq. Tuesday's announcement changes their destination. It was not clear whether the Pentagon would send another unit to Iraq in its place. If not, Iraq forces would drop by one brigade.
Ahead of his first foreign trip this week, Obama told a Canadian news organization that the United States will seek a more comprehensive, diplomatic approach to Afghanistan, where the U.S. has been engaged in war since 2001.
"I am absolutely convinced that you cannot solve the problem of Afghanistan, the Taliban, the spread of extremism in that region solely through military means," the president said in a White House interview with Toronto-based Canadian Broadcasting Corp.
Obama is scheduled to make a quick day trip to Ottawa on Thursday.
Obama agreed to a troop recommendation from Defense Secretary Robert Gates, the lone holdover from the Bush administration. Pentagon officials had been expecting a similar announcement for weeks, but the new Obama team took about a month choosing how and when to add forces to a war that has been sliding backward.
The president made his decision Tuesday, a senior White House official said. The official, who spoke on condition of anonymity ahead of the announcement, said Obama informed congressional leaders and Afghan President Hamid Karzai by phone.
Karzai's office said the two presidents talked about security issues and the Afghan presidential elections in August during their first conversation since Obama took office.
Karzai admitted last week that almost four weeks after Obama's inauguration he still had not spoken with the U.S. leader. Karzai spoke with former President George W. Bush regularly, fueling speculation that Obama was sending a clear signal that Karzai's standing with him was much lower.
The planned troop deployment does not preclude sending more forces in the future, the official said. Any others, however, would come as part of a broader strategic review of the entire policy in Afghanistan and Pakistan, not as a stand-alone troop decision, the official said.
That review should be completed sometime around the end of March, which coincides with a NATO summit in Europe.
The strategy review for the Iraq war is expected to be completed in about two weeks or so, with announcements expected then on troop drawdowns, the White House official said.
U.S. commanders have said they want to beef up the expeditionary units and trainers in Afghanistan's southern region with enough new troops to stem the violence without becoming an occupying force that would alienate the population.
McKiernan has asked for more mobile forces and believes having a Stryker brigade will allow soldiers to move more easily along the rugged trails to the widely dispersed tribal enclaves.
Stryker brigades come outfitted with several hundred eight-wheeled, 19-ton Stryker vehicles, which offer greater protection than a Humvee and are more maneuverable than the heavily armored mine-resistant vehicles that are being used across Iraq.
Below is a transcript of President Obama's full statement on the Afghan troop increase:
There is no more solemn duty as President than the decision to deploy our armed forces into harm's way. I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action. The Taliban is resurgent in Afghanistan, and al Qaeda supports the insurgency and threatens America from its safe-haven along the Pakistani border.
To meet urgent security needs, I approved a request from Secretary Gates to deploy a Marine Expeditionary Brigade later this spring and an Army Stryker Brigade and the enabling forces necessary to support them later this summer. This increase has been requested by General McKiernan and supported by Secretary Gates, the Joint Chiefs and the Commander of Central Command. General McKiernan's request for these troops is months old, and the fact that we are going to responsibly drawdown our forces in Iraq allows us the flexibility to increase our presence in Afghanistan.
This reinforcement will contribute to the security of the Afghan people and to stability in Afghanistan. I recognize the extraordinary strain that this deployment places on our troops and military families. I honor their service, and will give them the support they need.
This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires. That is why I ordered a review of our policy upon taking office, so we have a comprehensive strategy and the necessary resources to meet clear and achievable objectives in Afghanistan and the region. This troop increase does not pre-determine the outcome of that strategic review. 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. 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.