U.N. Secretary-General Kofi Annan is preparing to tell the United States Thursday, in a speech to the General Assembly, that only the United Nations can authorize the use of force against Iraq. In his own speech to the world body, President Bush is expected to present his case against Saddam Hussein, who Washington says still has weapons of mass destruction in defiance of U.N. resolutions.

Mr. Annan will stress his favorite theme, multilateralism, the principle the United Nations was built on. The secretary-general will not mention possible U.S. plans to attack Iraq. But he will make clear that the situation in Iraq is a problem for the United Nations to fix, or at least to provide legitimacy for a course of action. That would mean approval by the Security Council.

The secretary-general will also warn that Washington's leadership in the world would be weakened if it chooses to act without the blessing of the United Nations.

The U.N. took the unusual move of releasing Mr. Annan's speech before he delivers it, in time for the early news cycle, fearing that it would be overshadowed by the much-anticipated remarks of President Bush less than an hour later.