US President Donald Trump says he has reached an agreement with Russian President Vladimir Putin allowing Moscow to "immediately release" diesel into the global market.

He said Putin had agreed to release 300,000 tonnes of diesel immediately, followed by 500,000 tonnes in November and another million after that.

"Diesel Prices for Americans and, indeed, the World, will be COMING DOWN, IN RECORD NUMBERS, AND FAST!" Trump wrote on Truth social.

Diesel prices in the US have doubled since the war with Iran began in February, putting pressure on Republicans ahead of the midterm elections over the rising cost of living.

The average diesel price in the US is currently $6.28 a gallon, according to the AAA, down from the record high of $6.53 recorded at the the end of September.

Shortly after Trump's announcement, the US Treasury formally issued a temporary license allowing Russian diesel into the market.

An envoy for Putin, Kirill Dmitriev, welcomed the deal on social media, saying: "Russia-US co-operation on diesel and energy will benefit the world."

It is the latest effort from the president to lower fuel prices in the US.

Earlier this week, Trump said he was "thinking about" suspending the federal tax on gasoline, and also announced that he would allow so-called red dye diesel to be used on US highways without facing federal tax.

He has also successfully pressured G7 nations into releasing 100 million barrels of oil and diesel from stockpiles to help ease supply concerns, and previously supported calls for a ban on diesel exports from the US.