WASHINGTON - Despite deep misgivings among administration hard-liners, President Bush decided yesterday to seek a multinational force for Iraq under a United Nations mandate and to call on the world body to play a major role in forming a new Iraqi government, a senior administration official said.

Bush's decision is likely to bring about a significant change in Iraq, with American occupation authorities yielding some power to U.N. officials, and France, Germany and Russia, which had opposed the U.S.-led invasion, gaining influence over Iraq's future.

The decision was disclosed after a meeting yesterday between Bush and Secretary of State Colin L. Powell, who had sought authority to seek a new U.N. mandate on Iraq.

"The president authorized the secretary today to go forward with discussions with his counterparts" among U.N. Security Council members, the senior Bush administration official said. The aim is to "look for a resolution that can define further how the U.N. can support the Iraqis in the political process and to move forward with a multinational force authorized by the U.N."

The move to seek greater international involvement follows a series of violent setbacks to the U.S.-led occupation in Iraq, including four recent bombings that have killed scores of people and a mounting death toll among American troops.

And the chief U.S. administrator in Iraq, L. Paul Bremer III, has warned that it will cost "several tens of billions" of dollars to rebuild the war-torn nation.

Countries such as India, Pakistan and Turkey have made it clear that they would send troops to Iraq only under a new U.N. resolution that would give them an international imprimatur and not make them appear to be part of a U.S. occupation force. Such a mandate is also seen as the key to gaining greater support from the Arab world.

Prospective financial contributors, such as Germany and France, demand that the United Nations be given greater authority over Iraq's future, which would diminish the freedom of action of the United States.

The U.N.-authorized force is expected to remain under American command, with U.S. troops forming the largest contingent. The United States has about 140,000 troops in Iraq.

Bush's decision overcame deep divisions within his administration that echoed past debates between pragmatists, mostly in the State Department, and hard-liners in the Pentagon and elsewhere.

While State Department officials had pushed for a U.N. resolution that would bring more international participation in Iraq, hard-liners had expressed serious misgivings about giving the United Nations a wider role.

As recently as Aug. 20, White House spokesman Scott McClellan had discouraged speculation that the United States would give up control over the reconstruction of Iraq.

"This is a coalition-led effort, and it has been from the beginning," McClellan told reporters, referring to the U.S.-led coalition that includes Britain and smaller European allies. "The coalition will continue to lead that effort, but we appreciate all the help we are receiving from other countries."

In Washington, some officials recall nightmares of the early 1990s, when then-U.N. Secretary-General Boutros Boutros-Ghali persuaded the United States to join a disastrous nation-building exercise in Somalia. U.S. troops became targets of the country's warlords, as did U.N. troops from Muslim nations such as Pakistan.

The officials also recall a period of "dual-key" command during the war against Bosnian Serbs in 1994 and 1995 that allowed U.N. officials to block offensives by U.S. and NATO forces.

U.S. officials hope that past military problems can be avoided by placing the multinational force under American command, an idea first broached publicly last week by Deputy Secretary of State Richard L. Armitage.

But some U.S. officials remain wary of handing the United Nations a major role in the development of Iraq's new government and economy, fearing that would undercut Bush's goal of transforming the Middle East, with Iraq eventually serving as a beacon of reform and moderation.

"The vision we have is of a democratic, not a U.N.-ized Iraq," said a senior administration official who has played a key role in internal debates.

Among conservatives in the Bush administration, particularly in the Pentagon, the United Nations is identified with costly oversight, a strangled decision-making process and a long-term presence that hampers the development of democratic institutions.