CAMARILLO, Calif. (CNN) - Gas prices rose nearly four cents during the past two weeks to a national average of $1.56 per gallon, a survey said Sunday.

That was an increase of 3.85 cents, said Trilby Lundberg, publisher of the Lundberg Survey.

The survey of nearly 7,000 gas stations across the country was carried out July 25 and Aug. 8.

The 2.5 percent increase in prices coincided with a 6.7 percent increase in the price of crude oil -- from $30.17 per barrel to $32.18 -- over the same two-week period.

Though the national average is more than 14 cents higher than what it was a year ago, on Aug. 9, 2002, it is more than 16 cents less than what it was at the height of concerns over the U.S.-led invasion of Iraq this spring. Prices peaked March 21 at $1.73 per gallon.

Lundberg said this latest increase is likely due to continued unrest in Nigeria and Indonesia and delays in getting Iraq to get its oil production up to speed.

Gas was most expensive in Honolulu, at $1.96, and cheapest in Charleston, S.C., where it cost $1.39, Lundberg said.

Here are prices in other cities: Dallas, $1.48; Minneapolis, $1.65; Seattle, $1.60; Chicago, $1.68: Boston, $1.58; Philadelphia, $1.53; Louisville, Ky, $1.57; Atlanta: $1.47.