ABU GHRAIB, Iraq — The United States handed over control of all combat duties to Iraqi security forces on Saturday in a further sign that its withdrawal is on track despite a political impasse in Iraq and a recent rise in violence.

President Barack Obama said last Monday he would stick to his promise to end U.S. combat operations in Iraq by August 31, with security being left in the hands of Iraq's own U.S.-trained army and police.

"Today is an extremely important day as we continue to progress toward turning over full responsibility to the Iraqi security forces," General Raymond Odierno, top commander of U.S. forces in Iraq, told reporters after a departure ceremony for the last U.S. combat brigade.

Seven years after the U.S.-led invasion to topple Saddam Hussein, Washington is reducing the number of troops in Iraq to 50,000 by Sept. 1 from just under 65,000 currently and close to 150,000 during the height of the conflict.