The Reuters/Ipsos poll, conducted Friday through Sunday, found that 69 percent of Americans -- including four in 10 Republicans -- think US President Donald Trump has not "clearly explained the goals of US military involvement in Iran."
Approval of the conflict has been below 40 percent since the United States and Israel launched joint strikes on Feb. 28.
A growing number of polls have shown rising public backlash against the war. A recent poll released by The Washington Post-Ipsos found that 68 percent of Americans believe the war is "not worth fighting."
Trump said Monday that he had decided to pause US strikes on Iran to give negotiations another chance, while threatening that he could order a restart of strikes if talks with Iran fail.
US Defense Secretary Pete Hegseth asked for more funding for the war at a Senate hearing last week, where he said the war has cost 37.5 billion US dollars so far, up from the 29 billion dollars estimated in May.