The letter - signed by Senate Minority Leader Tom Daschle, former first lady Hillary Clinton, Ted Kennedy from Massachusetts - also questioned if there were adequate numbers of US troops in Iraq.
"The shocking deaths of four American security contractors in Fallujah have revealed the growing role that private security contractors are playing in Iraq," the letter read.
It said contractors, often former soldiers, operate in a fashion similar to special forces, but they are not under US military control and not subject to rules governing the conduct of American forces.
"It would be a dangerous precedent if the United States allowed the presence of private armies operating outside the control of governmental authority and beholden only to those who pay them," the letter said.
"In the context of Iraq, unless these forces are properly screened by United States authorities and are required to operate under clear guidelines and appropriate supervision, their presence will contribute to Iraqi resentment."
It said that such delegating raised "serious questions."
"The presence and number of these private security personnel again raise the question of the adequacy of United States troop levels in Iraq," it stated.
It closed by requesting a full tally of the number of privately armed non-Iraqi security personnel operating in Iraq, and urged Mr Rumsfeld to adopt written guidelines for their use.
-- AFP
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