The four-day poll showed just 31pc of Americans support US

The four-day poll showed just 31% of Americans support US military action against Iran, down from 37% in a March /Ipsos poll and 34% earlier this month.

The decline was driven by fewer self-identified Republicans backing the conflict.

Some 69% of Republican respondents support the war, compared with 77% in March.

The war has been dragging on Trump's public standing this year, and for the second survey in a row just 33% of respondents said they approved of the Republican's performance in the White House, the lowest level in /Ipsos polls in Trump's first or second term.

Trump has - which started with US-Israeli attacks on Iran on February 28 - is necessary to keep Tehran from developing a nuclear weapon.

The conflict has cooled in recent months but Iran has kept in place a de facto blockade of oil exports from the region, which has pushed US gasoline prices near record highs.

Trump has vowed to ramp up economic pressure on Iran and Treasury Secretary Scott Bessent on Monday announced a possible expansion of sanctions on countries doing business with the country, but stopped short of actually imposing penalties, a sign of how thorny the conflict has become for the administration. Fears of a long war.

Some 83% of the country thinks the war will go on "for an extended period of time," the latest /Ipsos poll found, up from 80% earlier this month. Trump campaigned ahead of the 2024 presidential election on promises to control inflation and avoid lengthy military conflicts.

Six months into the war with Iran, US gasoline prices are more than a dollar higher per gallon than they were before the war.

That is weighing on Trump's Republican allies who will be defending narrow congressional majorities in the November 3 midterm elections. Independent voters in the poll favoured Democrats over Republicans - 33% to 19% - when asked who they would vote for in congressional elections if the election were held today. The poll surveyed 1,215 US adults nationwide and had a margin of error of 3 percentage points in either direction.