President Barack Obama will announce in his State of the Union speech later 34,000 US troops will leave Afghanistan over the next year, US media say.
The move would effectively cut in half the current US troop levels in the country from about 66,000.
The US will end combat operations in Afghanistan by the end of 2014, but it is unclear how large a residual training force will remain behind.
Mr Obama could also use his speech to address post-2014 troop levels.
The US president and his Afghan counterpart, Hamid Karzai, agreed in January 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. But administration officials have also said they would consider leaving no troops in the country.
