Four-dollar gas is back.

AAA says the national average price for a gallon of regular gasoline reached $4.00 on Monday.

It marks the second time gas prices have climbed to the $4 mark during the conflict between Iran and the United States.

Hopes for a long-term ceasefire had helped ease prices in recent weeks. However, renewed military strikes between the U.S. and Iran have raised fears of a broader regional conflict, pushing fuel prices higher.

AAA says the national average for diesel has climbed to $5.11 a gallon.

Experts warn higher diesel prices can increase transportation costs, putting additional inflationary pressure on goods delivered by semi-trucks across the country.

The jump in fuel prices comes after Brent crude oil briefly topped $90 a barrel overnight.