US President Barack Obama is sending 1,500 more troops to Iraq, almost doubling the current deployment.
The White House, which said the new forces would not be in combat, also asked Congress for $5.6bn (£3.5bn) to fund the mission against Islamic State in Iraq and Syria.
The request includes $1.6bn for an “Iraq Train and Equip Fund”.
The new troops will support the roughly 1,600 US personnel already in Iraq, under what the Pentagon calls Operation Inherent Resolve.
The US says its troops in northern Iraq have no fighting role as they help to train national and Kurdish forces to battle the Islamic State jihadists.
“As a part of our strategy for strengthening partners on the ground, President Obama today authorized the deployment of up to 1,500 additional US military personnel in a non-combat role to train, advise and assist Iraqi Security Forces, including Kurdish forces,” said the White House.
Press secretary Josh Earnest said the new troops would operate at military facilities beyond Baghdad and Irbil, where they have hitherto been based.
Pentagon press secretary Rear Adm John Kirby said the military would set up several training sites across Iraq to instruct 12 Iraqi brigades.
It would also establish two operations centres where small advisory teams can work with Iraqi forces at headquarters and brigade levels.
One of those hubs is expected to be in Anbar province, where the extremists have reportedly been slaughtering men, women and children.
The latest deployment is based on an assessment of Iraqi security forces made by US Central Command, said the Department of Defense.
On Friday morning, President Obama hosted members of Congress at the White House, where he discussed the fight against Islamic State, among other issues.
House Speaker John Boehner reportedly told Mr Obama that Republicans would work with him next year to grant a new authorisation for military force against the jihadists.
Mr Obama’s decision comes three days after his party suffered bruising losses in the midterm elections, a result driven in part by the President’s unpopularity.
The US has steadily ramped up its mission in Iraq since Mr Obama announced in June that 300 military advisers would be sent back to the country, three years after American forces withdrew.
The Obama administration is also carrying out airstrikes against Islamic State militants in Iraq and Syria, though the Pentagon has acknowledged such raids alone will not be enough to stop the group.
Last week, Iraqi forces recaptured a town from the extremists. Last month, Islamic States forces lost a string of towns near the Syrian border.
Earlier this week, the UK said it would step up its military presence in Iraq as it seeks to train local ground forces to take on the jihadists.