Two out of three Americans think US President Donald Trump has not clearly explained why the country has gone to war with Iran, according to a Reuters/Ipsos poll completed on Monday that also showed his approval rating ticking up from the lowest level of his term.

The four-day poll revealed deep concerns about surging gasoline prices, and also suggested many voters are casting blame for their troubles on Trump's Republican allies, who will be defending their congressional majorities in the November midterm elections.

More than two months into a conflict that began February 28 with a US-Israeli bombing campaign, some 66% of poll respondents - including one in three Republicans and almost all Democrats - said Trump has not "clearly explained the goals of US military involvement in Iran."

Some 63% of the country says their household's personal financial situation has taken a hit from recent gas price increases, up from 55% in a Reuters/Ipsos poll conducted March 17-19.

Some 36% of Americans approve of Trump's performance, up two percentage points since a late April Reuters/Ipsos poll showed his approval rating at 34%, which was the lowest level of Trump's current term in office.

The Reuters/Ipsos poll had a 3-percentage-point margin of error in either direction, based on the number of people surveyed. The poll, conducted online and nationwide, gathered responses from 1,254 US adults.