The new US military commander in Iraq, Lt Gen Ray Odierno, has said that recent security gains there are "fragile and reversible".
He was speaking in Baghdad at a ceremony to replace Gen David Petraeus.
Gen Petraeus, who implemented the "surge" of nearly 30,000 extra US troops in Iraq, is widely credited with driving down levels of violence there.
He has been promoted to oversee operations in Afghanistan and in the Middle East, including Iraq.
Start your workday the right way with the news that matters most. Learn more