It is crucial for the Bush administration to find the weapons it argued were the key justification for going to war in Iraq.
The United States is claimed to be recruiting UN weapons inspectors
There were administration officials who were contemptuous of the United Nations' efforts to track them down.
But the head of the UN team responsible for chemical and biological weapons and missiles, Hans Blix, says the United States has approached several of his inspectors and asked them to end their contracts and join its own operation.
Well-informed sources have told me the same recruitment effort is being made with nuclear experts employed by the International Atomic Energy Agency.
People in both UN bodies are said to be infuriated.
Hans Blix and Mohamed ElBaradei say only the UN is authorised to inspect
The head of the International Atomic Energy Agency, Mohamed ElBaradei, has now emphasised that it remains the sole body with legal authority to verify Iraq's nuclear disarmament.
Mr ElBaradei said the agency expected to go back after the end of hostilities to resume its inspections. But that must be in doubt.
United States forces are already looking for weapons of mass destruction in the areas of Iraq they control.
Administration officials quoted by the Washington Post say an international entity may be allowed to verify discoveries after the fact.
But Dr Blix told the paper that his organisation would not accept being led, "as a dog", to sites chosen by the United States.
Any discoveries of forbidden Iraqi weapons announced by Washington are likely to be greeted with scepticism by a large chunk of international opinion, especially in the Arab world.
As Mr ElBaradei put it, credibility will come only through impartial, international inspections.