US President George W Bush is set to announce plans to withdraw about 8,000 troops from Iraq by February and to send additional forces to Afghanistan.
Mr Bush will say in a speech on Tuesday that the improving security situation in Iraq will allow a "quiet surge" of troops in Afghanistan in coming months.
A Marine battalion due to go to Iraq in November will be sent to Afghanistan, followed by an Army combat brigade.
There are currently 146,000 US troops in Iraq and 33,000 in Afghanistan.
Any long-term decision about their future deployment will be left to Mr Bush's successor, who will take office in January.
'Degree of durability'
The BBC's Jonathan Beale says the continued decline in violence in Iraq since last year's US troop "surge" has given President Bush a chance to ease the growing strain on his country's military.
Acting on the advice of his generals, Mr Bush will announce on Tuesday that a Marine battalion, comprising about 1,000 troops, scheduled to leave Anbar province in November will return home as planned without being replaced.
An army brigade of between 3,500 and 4,000 troops will also leave in February, accompanied by about 3,400 support forces, he will say.
"While the progress in Iraq is still fragile and reversible, Gen [David] Petraeus and Ambassador [Ryan] Crocker report that there now appears to be a 'degree of durability' to the gains we have made," Mr Bush will say in a speech at the National Defense University, according to the White House.
"And if the progress in Iraq continues to hold, Gen Petraeus and our military leaders believe additional reductions will be possible in the first half of 2009."
Our correspondent says the withdrawals announced on Tuesday will mark the start of a slow and limited draw-down based on what Mr Bush calls "return on success". However, it will still leave the bulk of US forces behind in Iraq.
Last month, Iraqi Prime Minister Nouri Maliki said that although a timetable for the withdrawal of the remaining troops did not exist, he had tentatively agreed with the US military to end the presence of foreign combat troops by 2011.
The Iraqi government is currently negotiating a security agreement on the future of US forces in Iraq before a UN mandate expires.
In his speech on Tuesday, Mr Bush will also signal that the US will make modest increases in the strength of its forces in Afghanistan to combat the growing threat posed by the Taleban.
Aid agencies point to a 50% increase in insurgent attacks in Afghanistan
"For all the good work we have done in that country, it is clear we must do even more," he will say.
"Unlike Iraq, it has few natural resources and has an underdeveloped infrastructure. Its democratic institutions are fragile."
"And its enemies are some of the most hardened terrorists and extremists in the world. With their brutal attacks, the Taleban and the terrorists have made some progress in shaking the confidence of the Afghan people."
In November, a Marine battalion that was scheduled to deploy to Iraq will instead go to Afghanistan. It will be followed in January by an army combat brigade.
The Agency Coordinating Body for Afghan Relief said last month that violence in Afghanistan had reached its worst level since 2001, when US-led forces overthrew the Taleban, with more than 260 civilians killed in July.