The national average U.S. retail price of diesel fuel rose to $1.745 from the previous average of $1.717, the Department of Energy reported May 10.

Once again, the highest prices in the nation were in California, where the average cost of diesel rose to $2.356 from the previous high of $2.274.

Truckers there have protested the steady increases at rail yards, interstates and at ports. A tentative agreement was reached last week, but it’s unclear whether further protests will continue.

Other prices included New England, at $1.768; $1.928 in the Rocky Mountain region; the Central Atlantic region, at $1.765; and on the West Coast, where the average price is $2.255.

Comparatively less stratospheric prices were found in the Lower Atlantic region, where the average price is $1.633; the Midwest, at $1.667; the Gulf Coast, at $1.645; and the East Coast, at $1.678

Meanwhile, Saudi Arabian Oil Minister Ali al-Naimi said an increase in supply was “essential” to ensure that global economic growth and demand for oil aren't hurt, Bloomberg reported.

OPEC should discuss at the upcoming International Energy Forum in Amsterdam an increase in quotas of at least 1.5 million barrels a day, or 6.4 percent, in the second half of the year, al-Naimi said.

“Oil hitting $40 a barrel on Friday was quite shocking, and maybe the Saudis suddenly thought that it was a bit worrying,” said Veronica Smart, an analyst at the Energy Information Centre, a consulting company in Newmarket, England. “OPEC are quite happy when oil is at $32 a barrel, but when it heads into $40 it becomes scary for them because people might start using less oil. They may be covering their backs with this move.”

OPEC ministers should issue a decision to increase quotas during their meeting on June 3 in Beirut, the Saudi minister said.

The last time prices reached Friday's levels was in October 1990, when the United Nations imposed an embargo on exports from Iraq. Saudi Arabia increased production by 2.6 million barrels a day to 8 million barrels between August 1990 and April 1991 to make up for the loss of output, leading to a decline in prices.

As the world's largest oil producer, Saudi Arabia can now spur output by 1.65 million barrels a day to bring its total to 10 million. Spare capacity from other OPEC members, excluding Iraq, is at about 1.6 million barrels for a total of 3.25 million, according to Bloomberg data estimates.

Oil prices have been above OPEC's target range of $22 to $28 a barrel since Dec. 1.