Washington DC - President Donald Trump on Monday again extended a waiver that allows foreign ships to carry oil and other commodities between US ports, as the war on Iran continues to disrupt energy flows and elevate costs.

The 90-day extension of relief from the Jones Act – which requires cargo between US ports to move on US vessels – comes shortly before an earlier waiver was due to expire in mid-August.

But the scope covered is narrower this time, focusing more on the transport of energy, a White House official confirmed Monday.

"Today, the Trump Administration issued a 90-day extension to the Jones Act waiver to ensure our military and key industries maintain uninterrupted access to critical resources," said White House spokeswoman Taylor Rogers.

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"Data shows the waiver has driven a significant increase in domestic deliveries of essential products such as gasoline, diesel, and jet fuel," she added.

The relief this time also calls for reviews on the availability of US vessels before determining if the waiver applies to individual voyages.

Oil prices have surged after US and Israeli strikes targeting Iran from late February sparked a war that has since engulfed the Middle East. Tehran retaliated by virtually closing off the Strait of Hormuz, a critical waterway for global energy transit.

Gasoline prices in the US have since jumped, straining household budgets as the key midterm elections draw close.

Under the 1920 Jones Act, cargo moved by water between US ports has to be transported on vessels that are US-built, US-owned and registered under the US flag. The aim was to promote American shipbuilding.

The Trump administration in March granted a 60-day waiver to temporarily lift this requirement, and extended relief in late April as economic fallout from the conflict persisted.

For now, negotiations on ending the war and reopening the Strait of Hormuz have stalled.