BAGHDAD (AFP) - US Defence Secretary Robert Gates met US and local commanders in Iraq on Monday, reviewing what he said will be "a mission transition" as the US force shrinks and turns over more of the country to Iraqis. Within minutes of his arrival, two car bombings in Baghdad killed at least 12 people and wounded 32, security officials said. Gates flew in unannounced to promote General Raymond Odierno to his fourth star and attend a military ceremony on Tuesday when he takes over command of the 146,000-strong US force in Iraq from General David Petraeus. With levels of violence down to around four-year lows after an 18-month "surge" in US forces, President George W Bush last week announced plans to send 8,000 troops home by January. "There is no question we will still be engaged as we are, but the areas in which we are seriously engaged will I think continue to narrow," Gates said. Responsibility for security of 11 of Iraq's 18 provinces has already been turned over to Iraqis and Gates said a couple more would probably join them by the end of the year. Odierno, an early proponent of the surge, implemented it as the corps commander in Iraq from December 2006 to March 2008, which Gates said made him the right person to replace Petraeus. Meanwhile, a woman suicide bomber blew herself up in a crowd of people during a feast in the Iraqi province of Diyala on Monday, killing at least 20 people and wounding around 30, a top military officer said. General Abdel Karim al-Rubaie, the commander of Iraqi troops in Diyala, north of Baghda, said the attack was in the town of Bala Druz. He said the bomber blew herself up when people had gathered to break the Ramadan fast at the house of a detainee who had been released from the US military prison Camp Bucca on Sunday. Baquba police Lieutenant Ali Ahmed also confirmed the attack, saying the detainee was released on Sunday and had arranged a special dinner on Monday for his friends and relatives when the attack took place.