Speaking to reporters after briefing the Security Council, Butler said some members asked whether there could have been a mixup on the test results.
"It is utterly unambiguous. These degradation products could be from no other substance. These are unique products. They were VX. They were found in a missile warhead. That is weaponization," he said.
Iraq has rejected a U.S. Army laboratory's finding that traces of a deadly VX nerve gas were on shards of Iraqi missile warheads, and is seeking independent analysis of the samples.
Butler briefed the Security Council Wednesday about results of an analysis by the U.S. Army laboratory at Aberdeen, Md.
Diplomatic sources said the lab reported finding traces of VX and a stabilizer on Iraqi warheads, despite Iraq's claims that it never succeeded in transforming the deadly nerve agent into a weapon.
CBS News Correspondent Jim Stewart reports that the discovery shows the Iraqis were further along in biological warfare than previously thought. And if they can stabilize the nerve gas, they can store it, hiding it from inspectors and bringing it out unexpectedly for use in weapons.
The finding could undermine efforts by Russia, France, and China to close the seven-year inspection program and lift crippling economic sanctions imposed on Iraq in 1990 after Saddam Hussein's forces invaded Kuwait. The invasion led to the 1991 Persian Gulf War.
In a statement Tuesday, Iraq's U.N. mission said Baghdad rejects the results, adding they "cannot be accurate since VX was not used in any kind of munitions in Iraq due to continuous production failure."
The Iraqis said the samples were too small to be conclusive and "finding a trace of a stabilizer in one sample out of seven analyzed is not evidence because, if VX was used, a stabilizer would have been found in every sample."
Iraq said the results must be verified by laboratories in a neutral country or in several countries.
VX is a colorless, odorless liquid that turns into a gas when it comes into contact with oxygen. A few drops can kill in minutes.
Despite the American report, the Security Council is deeply divided over the Iraq issue. The United States and Britain insist Iraq be compelled to provide full disclosure of albanned weapons before sanctions are lifted.
Russia, China, and France are equally resolved that the inspections should wrap up as quickly as possible, a position also held by chief advisers to Secretary-General Kofi Annan.
It remains unclear whether the new evidence would be enough to persuade the Russians, French, and Chinese to change their position.
Iraq has maintained it never produced VX warheads or figured out how to store the deadly nerve agent in bulk. Findings to the contrary would call into question Iraq's previous declarations about the state of its programs of mass-destruction weapons.
In Baghdad, Deputy Prime Minister Tariq Aziz said that 1.7 tons of VX had been produced, but that it was not of weapons grade. U.N. officials suspect the Iraqis produced up to four tons and that if some was placed in warheads more could be hidden in Iraq.