WASHINGTON - The United States will, by the end of 2008, have spent $100b on contractors in Iraq since the invasion in 2003, according to a congressional report to be released Tuesday. It is a milestone that reflects the United States' unprecedented level of dependence on private firms in the war, said sources familiar with the report. The report, addressed by the Congressional Budget Office, says that one out of every five dollars spent on the Iraq war has gone to contractors for their services to the US military and other government agencies, in a war zone where employees of private firms now outnumber American troops. The Pentagon's reliance on outside contractors is proportionately far larger than in any previous periods, and it has fuelled rebuke that the outsourcing has led to over billing, fraud and shoddy and unsafe work that has endangered or even killed American troops. Legal and political questions were also aroused about whether the United States has become too dependent on the armed private security contractors on the 21st century battlefield. The report said from 2003 to 2007, the government-awarded contracts in Iraq worth about 85 billion dollars, and that the administration was now awarding contracts at a number of $15b to $20b a year. At that pace, contracting costs will surge past the mark of $100b before the end of the year.