WASHINGTON, Oct. 16 — Defense Secretary Robert M. Gates is pressing for the nearly 10,000 armed security contractors now working for the United States government in Iraq to fall under a single authority, most likely the American military, in an effort to bring Blackwater USA under tighter control, senior administration officials and Pentagon advisers say.

That idea is facing resistance from the State Department, which relies heavily for protection in Iraq on some 2,500 private guards, including more than 800 Blackwater contractors, to provide security for American diplomats in Baghdad. The State Department has said it should retain control over those guards, despite Blackwater’s role in a September shooting in Baghdad that exposed problems in the current oversight arrangements.

In practical terms, placing the private security guards who now work for the military, the State Department and other government agencies under a single authority would mean that those armed civilians would no longer have different bosses and different rules. Pentagon advisers say it would also allow better coordination between the security contractors and American military commanders, who have long complained that the contractors often operate independently.

Mr. Gates has not publicly stated his final position on any reorganization, but his thinking on how to manage security contractors was described by administration officials, military officers and outside advisers to the Defense Department, who spoke on condition of anonymity.