The letter warned Butler that he must assume responsibility for authorising surveillance flights "especially in the circumstances in which our anti- aircraft artillery is open everywhere in anticipation of possible aggression," the sources said.
The US Ambassador to the UN, Bill Richardson, called the Iraqi threat an "irresponsible escalation" of the crisis, and a threat to escalate the confrontation into a military showdown.
"This is a direct threat on the United Nations," Mr Richardson told reporters following a meeting at UN headquarters. "A direct military threat to the United Nations."
Earlier, President Saddam called for dialogue to resolve the stand-off, the official Iraqi news agency (INA) said. He said Iraq wanted "a clear and complete picture" of when the UN would lift all of its sanctions against Iraq.
Speaking of the UN inspectors, he added: "If they have a question here or there, these questions and explanation have lasted for seven years and maybe they will last more ... the bulk of them are silly and provocative." Iraq has already prevented three US inspectors from entering the country from Bahrain.
In the hope of defusing the crisis, Kofi Annan, the UN Secretary General, is sending a team to Iraq.
Newt Gingrich, the House Speaker, said the message must be sent "that the United States will take whatever steps are necessary to enforce an ability to inspect". He said that "absolutely" included military strikes.Reuse content