President Obama speaking on Canada's CBC: 'Afghanistan is still winnable'
President Barack Obama has authorised the deployment of up to 17,000 extra US troops to Afghanistan.
Mr Obama said the soldiers had been due to go to Iraq but were being redirected to "meet urgent security needs".
It is the first major military decision by the Obama administration, and comes amid a major review of US policy in Afghanistan and Pakistan.
On Tuesday, the UN said that 2,118 civilians were killed in the conflict in 2008 - an increase of 39% from 2007.
Militants were to blame for 55% of the deaths, while US, Nato and Afghan forces were responsible for 39%, the UN said.
Mr Obama telephoned Afghan President Hamid Karzai to inform him of the troops increase.
The new US administration has been highly critical of Mr Karzai's government, and it is believed to be the first direct conversation the pair have had since Mr Obama's election.
The additional troops are to be sent before warmer weather brings an expected increase in fighting in Afghanistan, US defence officials said.
During his election campaign Mr Obama promised to withdraw US combat troops from Iraq within 16 months after becoming president, and make Afghanistan his priority.
In his latest statement, Mr Obama said: "The fact that we are going to responsibly drawdown our forces in Iraq allows us the flexibility to increase our presence in Afghanistan."
Afghanistan had "not received the strategic attention, direction and resources it urgently requires", he said.
A senior White House official was quoted by Reuters as saying that a decision on cutting US troop levels in Iraq would be taken within "weeks".
The new deployment will be made up of 8,000 marines, and 4,000 army soldiers, plus another 5,000 support staff. They will serve in the south of Afghanistan, where the violence has been worst.
"The 2nd Marine Expeditionary Brigade from Camp Lejeune, North Carolina, with approximately 8,000 marines will deploy to Afghanistan in late spring 2009," said a statement from the Pentagon.
"The 5th Stryker Brigade, 2nd Infantry Division from Ft Lewis, Washington, will deploy approximately 4,000 soldiers to Afghanistan in mid-summer 2009.
"Approximately 5,000 additional troops to support these combat forces will receive deployment orders at a later date."
The US already has about 14,000 troops serving with a Nato-led mission. There are also 19,000 US troops under sole US command charged with fighting Taleban and al-Qaeda insurgents.
US commanders in Afghanistan requested up to 30,000 additional troops last year, and former President George W Bush had repeatedly promised that more troops would be sent.
Analysts say Mr Obama's announcement is largely making good on the promises of the previous administration, rather than sending additional troops to those promised by Mr Bush.
The increase "is necessary to stabilise a deteriorating situation in Afghanistan", Mr Obama said.
"The Taleban is resurgent in Afghanistan, and al-Qaeda supports the insurgency and threatens America from its safe-haven along the Pakistani border."
"I recognise the extraordinary strain that this deployment places on our troops and military families."
* Allocation of new deployment between Operation Enduring Freedom and Nato ISAF to be confirmed.