IRAQ - The United States handed over control of all combat duties to Iraqi security forces on Saturday in a further sign its withdrawal is on track despite a political impasse in Iraq and a recent rise in violence. U.S. 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 towards 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 September 1 from just under 65,000 currently and close to 150,000 during the height of the conflict. During the ceremony, a division of the Iraqi army demonstrated vehicle checks and other security measures often employed in the war-battered country, where bombings and other attacks remain daily occurrences although overall violence has subsided since the conflict peaked in 2006-7. While this was the last combat brigade to hand over control to Iraqi forces, there will still be six brigades left in the country after U.S. combat troops leave by the end of the month. The six Advice and Assist Brigades, which come into effect from September 1 when the U.S. moves formally into an advisory role, will train and support Iraq's army and police. However, the inability of Iraq's fractious parliamentary parties to come up with a coalition government five months after elections has created a power vacuum potentially vulnerable to sectarian tensions and left many Iraqis concerned about the ability of local forces to maintain security.