In his first major military move, President Barack Obama approved the deployment of 17,000 more troops to Afghanistan, saying they were needed "to stabilize a deteriorating situation."
"There is no more solemn duty as president than the decision to deploy our armed forces into harm's way," Obama said in a statement.
"I do it today mindful that the situation in Afghanistan and Pakistan demands urgent attention and swift action."
Obama said the deployment orders were in response to a months-old request by the US commander in Afghanistan, General David McKiernan, who had asked for 30,000 more troops.
"To meet urgent security needs," Obama said he had approved a request by Defense Secretary Robert Gates to deploy a Marine Expeditionary Brigade in the spring, and an Army Stryker Brigade and support forces later this summer.
The White House said some 17,000 troops will be deployed to Afghanistan ahead of the Afghan elections scheduled for August 20, significantly building up the 38,000 US force already on the ground battling a growing insurgency.
Asked about possible future troop deployment orders, a senior administration official told AFP they were unlikely to come before an ongoing comprehensive review of US strategy in Afghanistan and Pakistan was completed.
"The thinking is that this was necessary at this time because of the expected increase of fighting in the spring and the upcoming elections," the official said on condition of anonymity.
"The strategy needs to be complete to determine whether additional troops would be necessary," the official added, saying the objective was for the review to be completed before a NATO summit in April.
The US-backed government in Kabul has come under intense pressure as the insurgency led by Taliban and Al-Qadea Islamic militants has gained strength and spread from the east and south into parts of the west and areas around the capital, Kabul.
As the security situation in Iraq has improved, the US has increasingly shifted its focus to the insurgency in Afghanistan and reduced the number of its troops in Iraq.
Last year saw the deadliest Taliban violence, including suicide attacks, assassinations of government officials and ambushes on Afghan and international troops.
"The Taliban is resurgent in Afghanistan, and Al-Qaeda supports the insurgency and threatens America from its safe haven along the Pakistani border," Obama warned.
"This increase is necessary to stabilize a deteriorating situation in Afghanistan, which has not received the strategic attention, direction and resources it urgently requires."
The Pentagon said Gates had ordered the deployment of two additional combat units totaling more than 12,000 troops, with Pentagon spokesman Bryan Whitman indicating they would be deployed in the violence-plagued south.
Another 5,000-some support troops would receive deployment orders at a later date "to support these combat forces."
Under the orders, some 8,000 Marines from the 2nd Marine Expeditionary Brigade will deploy to Afghanistan in late spring 2009, and about 4,000 soldiers from the 5th Stryker Brigade, 2nd Infantry Division would deploy in mid-summer 2009, the Pentagon said.
Obama indicated that the units being sent to Afghanistan had initially been earmarked for Iraq, saying the drawdown of US forces there "allows us the flexibility to increase our presence in Afghanistan."
He said the deployment decision would not predetermine the outcome of the review but instead "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 ordered the review amid growing alarm about mounting Islamic extremism in the region seven years after the United States launched its "war on terror" and ousted Afghanistan's Taliban regime.
It was unclear how the additional troops would be used, amid warnings that there is no military solution to Afghanistan's growing problems.
Senator John McCain, Obama's rival for the presidency in last year's election, welcomed the move but expressed hope that it was "just the first step in a new comprehensive approach to Afghanistan."
"A major change in course is long overdue," he said.
US intelligence has warned that endemic corruption in Afghanistan and the government's inability to deliver services and protect the population has eroded its legitimacy.