Four in five Americans expect ‌the US war with Iran to drag on for an extended period, according to a Reuters/Ipsos poll conducted as fighting escalated and President Donald Trump declared a blockade of Iranian shipping in the Gulf.

The three-day poll ​completed on Sunday found that 79% of respondents think US military involvement in ​Iran will “go on for an extended period of time,” up from 65% ⁠in late March.

Only 18% of respondents said they think the war would “end pretty quickly ​in a matter of weeks.”

Some 37% of respondents approved of US military strikes against Iran, ​which Washington resumed on June 26 in response to what it said were Iranian attacks on commercial shipping in the Strait of Hormuz.

The poll surveyed 1,019 US adults nationwide and had a margin of error of about ​four percentage points.

Trump said on Monday the United States was reinstating its blockade of Iranian ​shipping in the Gulf and would be reimbursed 20% on all cargo transiting the strait after Tehran said it had ‌closed ⁠the vital waterway and the two sides exchanged more missile and drone attacks.

The latest hostilities cast further doubt on an interim deal signed last month to reopen the strait and halt the war while the sides pursued 60 days of further negotiations.

Trump has said he considers the ​ceasefire over, while leaving ​the door open ⁠to further talks.

Sixty per cent of poll respondents said they expected gasoline prices to worsen over the next year as a result of the ​war.

Half said they believe the war has not been worth ​its costs.

Trump’s approval rating ⁠has hovered near the lowest levels of his political career since the conflict began, with Republican strategists warning that rising living costs have neutralised the political benefits of his tax cuts.

Higher gas ⁠prices and ​cost of living concerns pose a political risk to ​Trump’s Republican Party ahead of November’s midterm elections, in which it risks losing its House majority and possibly the ​Senate as well.