US President Donald Trump has said he does not think the Iran war will end until after November's midterm elections, adding that oil prices will not come down until then.

"I think the war's going to end immediately after the election because they can't hold out any longer," Trump said of Tehran on Wednesday.

He made the comments as he headed to Texas for the party's first-ever midterm election convention - and repeated it there, arguing that short-term economic pain was worth it to prevent Tehran from developing nuclear weapons.

Trump is facing falling polling numbers and pressure from within his Republican Party to end the war with Iran.

"Right after the election, oil prices are going to be tumbling downward," Trump told reporters. "I think it's going to take a little bit longer than the midterm."

He added that Iran was "desperate to try and affect the election, so that we can get a nice weak group of people in there" - an apparent reference to the Democrats - "and leave them alone and let them have their nuclear weapon".

He provided no proof of the election claim. Tehran has repeatedly denied it is seeking nuclear weapons.

At the convention in Texas, Trump echoed some of these lines, saying the price of oil would tumble "as soon as we win the war with Iran".

In his speech, he also referenced dissatisfaction among some Americans on the issue: "Guys come up to me and say, 'I wish you didn't do the war in Iran. Now gasoline is up'".

Trump also said further negotiations with Iran could possibly happen, "but it's not something we're looking at".

The US president initially predicted the war, which is now in its seventh month, would last only a matter of weeks. The conflict, and its knock-on effects on everything from fuel to groceries, has led to a steep decline in his approval ratings - and pressure from those within his party, including devout supporters, to quickly end the war.

Midterm elections will be held on 3 November, and Republicans are in danger of losing their control on Congress.

At the convention, Trump sought to energise the party ahead of November. Historically, a sitting president's party tends to lose seats in Congress in the midterm elections.

Fighting in the Middle East has intensified in recent days, as oil facilities throughout the region come under attack, driving up the global price of fuel and any item that requires shipping.

The price of oil hit $100 a barrel on Wednesday for the first time since July. Similarly, earlier this week the price of US diesel hit an all-time-high, which is likely to cause a ripple effect on prices across the US.

On Wednesday, the US military said it had attacked five oil tanker ships with ties to Iran. Tehran, in response, launched attacks on two US Navy destroyers and eight oil tankers in the Strait of Hormuz, causing "heavy damage" to the vessels, according to Iranian state media.

A day earlier, the Iran-linked Houthis in Yemen launched a wave of attacks in Saudi Arabia, sparking fires at oil facilities.

The fighting, which began again recently after a 60-day ceasefire between the US and Iran formally expired last month, has also raised predictions that the rising fuel costs could lead to higher interest rates in the US.