The national average price of a gallon of unleaded gasoline increased five cents to $2.61 for the week ending March 26, as fuel stations have begun off-loading their less-viscos winter blend for summer-blend gasoline, according to AAA.

Six states saw double-digit price increases, even as the U.S. Energy Information Administration reported a decline in demand of 1.7 million barrels per day to 9.3 million barrels per day.

The sharpest increases came in Idaho (13 cents), Utah (14 cents), Delaware (13 cents), New Mexico (12 cents) and South Carolina (10 cents).

"Right now we are seeing the market starting to purge winter-blend gasoline to make room for summer-blend," said Jeanette Casselano, AAA spokesperson. "The jump in gas prices is just the beginning for the season. AAA forecasts the national gas price average will be as much as $2.70/gallon this spring and summer."

The average price is now nine cents more than a month ago and 33 cents higher than a year ago.

States with the least expensive gasoline include Missouri ($2.31), Mississippi ($2.35), Arkansas ($2.36), Alabama ($2.37), Louisiana ($2.39), Texas ($2.39), South Carolina ($2.40), Oklahoma ($2.40), Tennessee ($2.41), and Ohio ($2.42).

Meanwhile, the average price of a gallon of diesel increased 3.8 cents to $3.01, which is 47.8 cents higher than a year ago, reports the EIA.