Senator Richard Lugar, the Republican chairman of the Senate Foreign Relations Committee, told BBC News Online's Talking Point programme that Security Council diplomats would have their "work cut out" for them when they met on Wednesday.
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On this occasion, Saddam will be disarmed
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Senator Richard Lugar
While noting that the first resolution had taken nearly two months of patient negotiation, he said he could not understand how any responsible state could veto a second.
"We will work, starting on Wednesday, with all other members of the UN Security Council to get a successful second resolution," he said.
Mr Lugar said Iraq still had a chance to avoid war if it accounted for its missing weapons of mass destruction (WMD) but, he stressed: "On this occasion, Saddam will be disarmed."
He said that without a statement from Baghdad on the fate of WMD identified by United Nations inspectors in the 1990s, the task of the inspectors now in Iraq was akin to searching for a needle in a haystack.
"The world needs to understand what has happened to the material found during the last inspections," Mr Lugar said, speaking in a live link-up from Washington on Sunday.
Evidence
Mr Lugar said he appreciated the continuing uncertainty among the US public about war with Iraq but the American administration had the support of the international community to tackle a state which posed a threat with WMD.
Taking questions from callers across the globe, the senator said he expected that more US intelligence on Iraq's arsenal would be revealed to the UN inspectors although there was always a risk of jeopardising sources within the country.
On the subject of inspectors interviewing scientists within Iraq, he said that Iraq's WMD installations were small, mobile facilities and could not be examined properly without full co-operation from Iraqi technicians on the ground.
He said he was speaking from personal experience of inspecting Russian facilities under bilateral agreements. He recalled seeing one plant producing soap which had been converted from anthrax manufacturing.
'Embarrassment'
He agreed that it was "embarrassing" that the West had not properly condemned Iraq for its use of WMD against Iran and its own Kurdish community in the 1980s, and he accepted that the West may have inadvertently helped Baghdad develop its WMD programmes.
The senator denied that the US was advocating a war to gain control of Iraq's oil reserves.
Every nation in the world, Mr Lugar said, would suffer from proliferation of WMD if the United Nations Security Council did not ensure Iraq's disarmament.