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 per cent of respondents think US military involvement in Iran will “go on for an extended period of time,” up from 65pc in late March.

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

Some 37pc 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.