The chief UN weapons inspector said on Saturday that coalition forces in Iraq apparently have not yet found any evidence of weapons of mass destruction.
Hans Blix told Swedish Radio that questions remained about chemical weapons protective suits and gas masks left by retreating Iraqi soldiers.
"One must ask how old these clothes are. They may have been there for a long time," Blix said. "In any case, they haven't found any weapons yet."
To prove that Iraq possess weapons of mass destruction "one has to find what the Americans call a smoking gun, or at least a gun," he said.
Blix supervised the UN weapons inspectors that entered Iraq in November and were pulled out of Baghdad shortly before the US-led military invasion on March 20.
The 74-year-old Swede also said British and US forces appeared to have underestimated their enemy.
"It's hard to avoid the impression that they are encountering much more resistance than they had counted on," he said. "It appears as if it will take longer than the military had assumed." Blix said he plans to step down from his assignment in June and return to research and writing.