The last US combat brigade pulled out of Iraq at dawn Thursday, a key milestone in the withdrawal of American forces more than seven years after the US-led invasion ousted dictator Saddam Hussein. Under the cover of dark, the 4th Stryker Brigade, 2nd Infantry Division, crossed the border into neighboring Kuwait ahead of the planned declaration of an end to US combat operations in Iraq by an August 31 deadline. Meanwhile, a New York Times report said Tuesday that the US State Department is to more than double the number of security contractors it employs in Iraq to around 7,000, filling a gap left by departing troops. The newspaper said the contractors would be deployed to defend five fortified compounds that will be left behind as US combat forces exit Iraq and the US mission switches from a military-led to a civilian-headed operation. Citing unnamed administration officials, the Times said private security contractors would operate radar to warn of enemy fire, search for roadside bombs, and fly surveillance drones. They could also staff “quick reaction forces” dispatched to rescue civilians in trouble. The massive increase in security contractors is an indication of the unusually large role that will be assumed by US diplomatic staff after combat troops leave Iraq. The Times said more than 1,200 specific tasks currently handled by US troops have been identified for handover to US civilians or Iraqis or to be phased out. The State Department meanwhile, seeking to outfit its employees for the next phase of their mission, plans to purchase 60 mine-resistant vehicles from the Pentagon and to expand its inventory of armored cars to 1,320. It also plans to add three planes to the sole aircraft it has now. – Agencies , and expand its helicopter fleet -- to be piloted by contractors -- to 29 from 17. The increased reliance on security contractors could cause conflict with Iraq's government, which is sensitive to the use of foreign security personnel because of their alleged involvement in incidents involving civilian deaths. But the forces employed by the State Department will not have immunity from Iraqi prosecution, will be required to register with the country, and will be trailed by State Department regional security officers for extra oversight. The pullout of the 4th Stryker Brigade leaves behind some 56,000 US soldiers who will gradually draw down over the coming year. It took two days for the 360 military vehicles and 1,200 soldiers to travel from Camp Liberty on Baghdad's outskirts and Camp Taji north of the capital, through the Shiite south, and into Kuwait, Bloom said. He said the remaining solders in the 4,000-strong brigade departed the country by air. The pullout coincided with the arrival of James Jeffrey, the new US ambassador to Iraq, who presented his diplomatic credentials Wednesday to the conflict-torn nation's head of state, President Jalal Talabani. Jeffrey's arrival comes during a political deadlock in Iraq, with no new government having yet formed after elections in March, and in the middle of the holy Muslim fasting month of Ramadan.