Barack Obama, the US president, will announce in his State of the Union speech that he will pull 34,000 US troops home from Afghanistan by early 2014, a White House official has said.
The long-awaited move will effectively halve the size of the current 66,000-strong US force in Afghanistan.
President Obama is not expected to address post-2014 troop levels in Tuesday's speech.
A senior US official said that Obama had telephoned Afghan President Hamid Karzai, British Prime Minister David Cameron and German Chancellor Angela Merkel to inform them of his decision.
A debate is also taking place within the administration on the size of a residual force, to train Afghan soldiers and to conduct "anti-terror" missions, that will remain behind after the formal withdrawal.
Last month,Obama and Karzai had agreed to speed the handover of combat operations to Afghan forces. The White House has previously signalled it favours keeping up to 9,000 troops in Afghanistan after the combat mission ends.
|US troop levels in Afghanistan peaked at
about 100,000 in 2010 [GALLO/GETTY]
The senior official said that Washington remained committed to a long-term strategic partnership with Afghanistan, and reiterated that talks on a bilateral security agreement were still taking place.
But administration officials have also said they would consider leaving no troops in the country.
Afghanistan has committed to taking full responsibility for its own security by the end of 2014 and the White House said there are now 352,000 troops in new Afghan security forces, in part due to a broad NATO training effort.
NATO says it will no longer lead combat operations in the next two years, but will provide support to Afghan soldiers.
Obama has made ending the wars in Iraq and Afghanistan the centrepiece of his presidency, and often declares a long decade or more of American war is almost over.
While Obama is expected to focus the majority of his address on the economy and job creation, foreign policy grabbed the spotlight after North Korea said it successfully detonated a nuclear device on Tuesday in defiance of UN warnings.
"The president will say that the only way North Korea will rejoin the world community is if they stop these threats and live up to their international obligations,'' said Tommy Vietor, a spokesman for the White House's National Security Council.