President George Bush has announced that about 8,000 US troops will be withdrawn from Iraq by February - with 4,500 being sent to Afghanistan.
In a speech at the National Defense University on Tuesday, he said
the United States faces "some of the most hardened terrorists and extremists in the world".
He said reduced violence levels in Iraq have allowed for a "quiet surge" of troops in Afghanistan.
There are currently 146,000 US troops in Iraq and 33,000 in Afghanistan.
President Bush announced that a marine battalion of about 1,000 troops, scheduled to leave Iraq's Anbar province in November, would return home as planned without being replaced.
An army brigade of between 3,500 and 4,000 troops will leave in February, accompanied by about 3,400 support forces.
A marine battalion due to go to Iraq in November will be sent to Afghanistan, followed by an army combat brigade in January.
Based on typical battalion and brigade strengths, this would total some 4,500 troops.
The Iraqi government is currently negotiating a security agreement on the future of US forces in Iraq before a UN mandate expires.
In signalling increases in the strength of US forces in Afghanistan, Mr Bush said the country faced "some of the most hardened terrorists and extremists in the world".