Half of all Americans believe the United States will go to war with Iran "within the next few years", according to a Reuters/Ipsos public opinion poll released on Tuesday amid increased tensions between the two countries.

While Americans are more concerned about Iran as a security threat to the United States now than they were last year, few would be in favour of a pre-emptive attack on the Iranian military.

But if Iran attacked US military forces first, four out of five believed the United States should respond militarily in a full or limited way, the poll, conducted between May 17 and 20, showed.

Nearly half – 49 per cent – of Americans disapprove of how Mr Trump is handling relations with Iran, the poll found, with 31 per cent saying they strongly disapprove. Overall, 39 per cent approve of Mr Trump's policy.

The survey showed that 51 per cent of adults felt the US and Iran would go to war within the next few years, up eight percentage points from a similar poll published last June. In this year's poll, Democrats and Republicans were both more likely to see Iran as a threat and to say war was likely.

Iran was characterised by 53 per cent of adults in the United States as either a "serious" or "imminent" threat, up six percentage points from a similar poll from last July. In comparison, 58 per cent of Americans characterised North Korea as a threat and 51 per cent characterised Russia as a threat.

Despite their concerns, 60 per cent of Americans said the US should not conduct a pre-emptive attack on the Iranian military, while 12 per cent advocate striking first.

If Iran attacked, however, 79 per cent said that the US military should retaliate: 40 per cent favoured a limited response with air strikes, while 39 per cent favoured a full invasion.