President Donald Trump has accused America's biggest oil companies of ripping off drivers at the gas pump - ordering the Justice Department to investigate after crude prices plunged but retail prices did not fall as quickly as he wanted.

In a furious post sent after midnight on Wednesday, Trump said oil companies were failing to pass on the full benefit of sharply cheaper crude to ordinary Americans.

'The big Oil Companies are not dropping their price at the pump commensurate with the sharply lower prices they are paying for Oil,' Trump wrote on Truth Social.

'Those prices are dropping like a rock! In other words, customers are being 'gouged.'

Trump said he had instructed the Department of Justice to act.

'I have instructed the DOJ to immediately start looking into this,' he wrote. 'Gasoline prices better start going down a lot faster than what I'm seeing!'

The president's late-night broadside came as drivers were already beginning to see relief after weeks of painful prices.

The national average price of gasoline fell for a sixth consecutive week, dropping 14.1 cents per gallon over the past week to $3.85 on Monday, according to GasBuddy.

That represents a decline of roughly 15 percent from the peak reached in May. 

Prices have fallen in most states, with particularly sharp drops in parts of the West and Midwest. 

GasBuddy data showed average prices fell by 25 cents a gallon in Colorado over the past week, 22 cents in Arizona and 21 cents in Ohio.

The average national price also slipped below $4 per gallon last week for the first time since March, after the US and Iran announced a memorandum of understanding aimed at ending their conflict and restoring shipping through the Strait of Hormuz.

But Trump made clear that he believes motorists should be seeing a faster and deeper drop at the pump.

Oil markets have shed much of the risk premium built up during the conflict with Iran, as fears of a prolonged disruption to the Strait of Hormuz - one of the world’s most important oil transit routes - began to ease.

Brent crude fell 1 percent to $76.30 per barrel Wednesday, while US West Texas Intermediate dropped 1.1 percent to $72.43 per barrel. 

Both benchmarks were at their lowest levels since early March after losing around 1 percent the previous day.

The retreat came as shipping through the strait showed signs of returning, albeit slowly. 

Two smaller crude tankers passed through the waterway on Monday, despite Iran saying it had again closed the passage over the weekend.