(Adds senior al Qaeda in Iraq leader killed) By David Clarke and Dean Yates BAGHDAD, Sept 28 (Reuters) - The first U.S. military unit scheduled to withdraw from Iraq under President George W. Bush's plan to cut troop levels has left the war zone. U.S. army officers say their stepped-up security drive around Baghdad is yielding results and led to a decline in the number of U.S. troop casualties this month. U.S. forces killed a senior leader of al Qaeda in Iraq, the military said. Brigadier-General Joseph Anderson, chief of staff for the Multi-National Corps-Iraq, described Abu Usama al-Tunisi as the "emir of foreign terrorists" in Iraq. A U.S. air raid in Baghdad killed at least eight people, medical sources said. A police source put the toll at 10 and said many were believed to be civilians. Bush increased troop levels by 30,000 this year to try to stem violence that was threatening to tear Iraq apart and to give the country's feuding politicians the "breathing space" needed to bridge their deep sectarian differences. Under pressure from opposition Democrats and senior Republicans for big cuts in troops, the president approved a plan from his top commander in Iraq to gradually reduce the U.S. force by 20,000 to 30,000 by mid-2008. Bush said improved security made the cuts possible. Captain Pamela Marshall, a military spokeswoman in Washington, said the 13th Marine Expeditionary Unit, a group of 2,200 Marines who were stationed in western Anbar province, had boarded a naval vessel and begun the trip home. A brigade combat team, which normally includes about 4,000 troops, is expected to leave Iraq in mid-December followed by four other brigade combat teams and two Marine battalions. The U.S. force now totals about 165,000 in Iraq. Anderson said Tunisi was killed on Tuesday south of Baghdad in an air strike. He said the militant, from Tunisia, had brought al Qaeda fighters into Iraq and led a group responsible for kidnapping U.S. soldiers in June 2006. "Abu Usama al-Tunisi was one of the most senior leaders within al Qaeda in Iraq," Anderson told Pentagon reporters by videolink from Iraq. He did not say which soldiers the group kidnapped. The Pentagon could not immediately provide that information. INVESTIGATION In Baghdad, a medical source at the Yarmouk Hospital said eight bodies had been brought in from a southern neighbourhood after U.S. helicopters targeted a building on Friday. It is the second time this week U.S. forces have been accused of killing civilians in air strikes. U.S. forces are investigating an attack in southern Iraq this week which local police said killed five women and four children. That strike took place on the same day and in the same area that Anderson said Tunisi had been killed. North of the capital, the Iraqi army said it had killed 30 suspected al Qaeda insurgents. Fifty-nine U.S. soldiers have been killed in September, according to the Web site icasualties.org which tracks military deaths, making it the least deadly month for U.S. troops since July last year. Twenty-two of the deaths were defined as "non-hostile", many of them road accidents. "What we found is that the current operations ... managed to disrupt a lot of (militant) cells," said a U.S. military spokesman, Lieutenant-Colonel Rudy Burwell. "We were able to push them from Baghdad and pursue them". "That's what we attribute the lower casualties to. "Obviously (the militants) have not been eliminated, but they have been disrupted." Shooting attacks and roadside bombs had been "trending downwards" since June, he said. September's figure is on track to be about half the death toll for May when extra U.S. forces deployed in greater strength into dangerous areas. (Additional reporting by Dominic Evans, Mussab Al-Khairalla, Aws Qusay, Haider Salahudeen and Kristin Roberts in Washington)
