ATLANTA (CNN) - Gas prices rose more than three cents per gallon over the past two weeks, to a national average of $2.33, a survey of U.S. filling stations found Sunday.

That's a nearly 20-cent-per-gallon increase in the price of gasoline since Dec. 2, according to the Lundberg Survey, which tallied prices Jan. 20 and Jan. 6 at about 7,000 stations in all 50 states.

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Further price rises over the short-term are likely, survey publisher Trilby Lundberg said.

"The gas market has been playing catch-up with the crude oil price increases and still has several pennies to go," she said.

Nervousness among oil traders over headlines coming out of three oil-producing countries -- Iran, Nigeria and Venezuela -- have increased upward pressures in the futures markets, she said.

Iran has resumed its pursuit of nuclear energy in a move that international leaders have interpreted as an attempt to build a nuclear weapon; the security situation in Nigeria has deteriorated; and Venezuela has tightened conditions for oil companies to do business in the country.

Prices have been further driven up by two recent government-mandated fuel reformulations and the requirement that corn-based ethanol be added to some gasoline, Lundberg said. Those steps make the process of refining gasoline more expensive, she said.

Upward pressures are likely to continue, Lundberg said. January is the year's lowest-consumption month for gasoline, and increased demand in coming months is expected to add to price pressures.

Drivers in Portland, Ore. paid the least, at $2.10 per gallon of self-serve regular, while Honolulu drivers paid the most at $2.80, the survey found.

Here are average prices of a gallon of self-serve regular in some other cities: Saint Louis, $2.16; Boise, Idaho, $2.19; Atlanta, $2.28; Dallas, $2.30; San Francisco, $2.39; Boston, $2.39; Philadelphia, $2.45; Long Island, N.Y., $2.61.

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